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A PRELIMINARY STUDY OF CLIMATIC 

CONDITIONS IN MARYLAND, AS 

RELATED TO PLANT GROWTH 



By 
FORMAN T. McLEAN 



Dissertation submitted to the Board of University 

Studies of the Johns Hopkins University in 

conformity with the requirements 

for the degree of Doctor of 

Philosophy 



BALTIMORE, 1015 

[Reprinted from Physioi-ooic\l Researches, Vol. 2, No. 4, Serial No. 14, FEBRrARY, 1917) 



A PRELIMINARY STUDY OF CLIMATIC 

CONDITIONS IN MARYLAND, AS 

RELATED TO PLANT GROWTH 



By l/^ 
FORMAN XrMcLEAN 



Dissertation submitted to the Board of University 

Studies of the Johns Hopkins University in 

confoi-mity with the requirements 

for the degree of Doctor of 

Philosophy 



BALTIMORE, 1915 

[Reprinted from Physioi.ooica.l Researches, Vol. 2, No. 4, Serial No. 14, February, 19171 



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A PRELIMINARY STUDY OF CLIMATIC CONDITIONS IN MARY- 
LAND, AS RELATED TO PLANT GROWTH ^ . 

(Carried out under the auspices of the Maryland State 
Weather Service, in 1914) 

FORMAN T. McLEAN 

ABSTRACT^ 

This study is an attempt to test certain methods for determining some of 
the quantitative relations between cHmatic conditions and the growth 
of plants. Since these relations are very complex, and since the interpre- 
tations of experimental results bearing on these relations are exceedingly 
difficult, the preliminary stages of such interpretations may be advanced 
by emplojdng the growth i-ates of a standard plant as a measure of the ef- 
fectiveness of the surroundings to produce growth. To do this, it is neces- 
sary to employ plants that are as nearly alike as possible at the beginning of 
the various tests. The plant is thus regarded as a sort of integrating and 
recording instrument, the reading of which is zero at the beginning of each 
observation period. The plant is allowed to grow during the period, and 
the effectiveness of the environmental conditions during that time is meas- 
ured in terms of the amount of growth produced. 

This method was employed in these studies, the plant used being soy- 
bean. A new observation period began approximately every two weeks 
and continued for a month, so that the different periods overlapped. Ob- 
servations on growth were also made at the end of about two weeks; that 
is, at the middle of the month. 

To have the plants of all tests nearly alike at the beginning of the period, 
they were always started from the seed. Dry seeds change less rapidly 
with time and are less influenced by surrounding conditions than plants 
in any other developmental phase. The growth here studied is thus that 
occurring during the first two weeks and during the first month, from the 
seed. 

At the beginning of each period, seeds were planted in plunged pots, the 
same soil being used for all stations, and the pots were furnished with auto- 



1 Botanical contribution from the Johns Hopkins University, No. 47. In the editing of this paper I have 

been assisted by Mr. F. M. Hildebrandt. — B. E. L. 

2 The manuscript of this paper was received July 1, 1916. This abstract was preprinted, without change, 
from these types, and was issued as Physiological Researches Preliminary Abstracts, vol. 2, no. 4, January, 
1917. 

129 

PHySIOLOGICAL. RESE.^RCHES, VOL. 2, NO. 4, 
SERIAL NO. 14, FEBRCARY, 1917 



130 FoRMAN T. McLean 

irrigators, to prevent the cultures from ever suffering from lack of soil mois- 
ture. The influence of rainfall, as it affects soil-moisture, was therefore 
removed from the main consideration. Temperature, evaporation and 
sunshine are thus the climatic conditions with which the study mainly deals. 
Onty two stations are here considered, Oakland (in the mountains of 
western Maryland) and Easton on the (eastern shore of Chesapeake Bay) . 
Evaporation was measured by means of standardized cyHndrical porous- 
cup atmometers. Daily maximum and minimum temperatures were ob- 
tained in the usual manner. Sunshine records are considered to some 
extent, as are also those of rainfall and soil-moisture. 

After about two weeks of growth from the seed, the following growth 
measurements were recorded: stem height, average number of leaves per 
plant, average length and width of mature leaves, and average of the prod- 
ucts obtained by multiplying length by width for each leaf. After about 
a month of growth, these measurements were repeated and, also, the aver- 
age leaf area and the average dry weight of tops per plant were determined. 
Apart from the study of various methods for growing and measuring the 
plants, measuring the climatic conditions and interpreting the data thus 
obtained, the present studies also yield some very definite indications 
regarding the interrelations holding between the various climatic features, 
on the one hand, and the manner and rate of development of the plants, 
on the other. These results are summarized below. 

1. Considering the entire period of observation at each of the two sta- 
tions here employed (which eml^raced nearly the entire frostless season at 
each station), the complex of environmental conditions experienced at the 
Easton station was much more efficient in producing growth of soy-bean 
plants than was the corresponding environmental complex experienced 
at Oakland. The three criteria mainly used in these studies for measuring 
plant growth — leaf area, stem height and dry yield of tops — all agreed in 
pointing to this conclusion. For the first two weeks of growth from the 
seed, the average daily growth increment in terms of leaf-product (the mean 
of the products obtained by multiplying the length by the breadth of each 
leaf) was 1.2 for the Easton season, and 0.9 for the Oakland season. The 
length of the season employed at Easton was 171 days, while the growing 
season began later and was terminated earlier at Oakland, where the length 
of season actually employed was 103 days. The total efficiency of the 
Easton season to produce plant growth, as in these tests, may therefore be 
regarded as proportional to 171 X 1.2, or 205.2, and the efficiency of the 
Oakland season may similarly be taken as proportional to 103 X 0.9, or 92.7. 
The total efficiency of the Easton season of observation (its power to pro- 
duce plant growth) thus appears to have been 2.21 times as great as was 
the corresponding efficiency of the Oakland season. About one month 
of the actual frostless season at Easton was not included in the season of 



Climatic Conditions in Maryland 131 

these studies, however, so that the total efficienc}^ of the Easton frostless 
season for 1914, measured in terms of leaf-product as here used, was about 
2.5 times as great as that of the Oakland frostless season. 

2. Of the five criteria by which the growth rates of the experimental plants 
were compared (stem height, total number of leaves, produced, leaf dimen- 
sions, leaf surface and dry weight), those of stem height, leaf surface and 
dry weight exhibited the greatest differences between different culture 
periods. The rates of growth in terms of leaf surface and in terms of dry 
weight varied in a similar manner with the same kind of variations in exter- 
nal conditions, while the growth rates measured in terms of stem elongation 
varied in another way with the same external differences. It thus appears 
that the rate of elongation of plant stems is influenced by external conditions 
differently from the rates of development of leaf surface and of dry weight 
for the same plants. In dealing with the quantitative relations of plant 
growth to external conditions it is therefore necessary to distinguish clearly 
between the various kinds of growth and the various criteria that may be 
employed in their measurement. 

3. The rates of growth in stem height were generally more rapid during 
the first than during the second fortnight of growth from the seed, for both 
stations. On the other hand, the rates of increase in leaf area (as approxi- 
mately measured by means of the leaf-product) were generally more rapid 
during the second fortnight. 

4. The growth rates generally showed very evident seasonal marches, 
by whatever criterion thej' were measured, increasing during the first part 
of the season and decreasing in the autumn. These seasonal marches were 
most apparent for the first two weeks of growth from the seed, and were 
most clearly shown by the rate of increase in stem height. They corre- 
spond, in general trend, to theseasonal marchesof the temperature conditions. 

5. The seasonal marches of both the growth rates and the temperature 
values for Oakland are quite markedly different from those for Easton. 
Both ranges are greater for Easton than for Oakland. The highest tempera- 
ture values and the highest growth rates occurred at Easton, and the grow- 
ing season was terminated by killing frost earlier at Oakland than at Easton. 
Nevertheless, the last two-week period before autumn frost at Oakland ex- 
hibited a higher temperature value and higher growth rates than did the 
last two-week period before frost at Easton. This difference between the 
magnitudes of the final minimum growth rates observed at the two stations 
appears to emphasize one of the main differences between a mild, equable, 
coastal climate and a much more rigorous mountain climate, as these may 
influence plant growth. In the milder climate of Easton, with its small 
daily range of temperature, the frostless season is apt to be prolonged until 
the growth of many plants is much reduced or entirely checked by low tem- 
perature. In the mountain climate of Oakland, however, with its large 



132 FoRMAN T. McLean 

daily range of temperature and high nocturnal radiation, very low night 
temperatures and frosts occur earlier in the season, while the day tempera- 
tures and the growth rates of many plants are still high. These differences 
between the two stations, as regards the temperatures and growth rates 
exhibited at the close of the season (just before autumn frost), are surely 
intimately associated with the two types of climate here illustrated, and are 
of undoubted importance in the consideration of plant life in general. An- 
other difference to be noted between the two stations here considered 
refers to the time of occurrence, within the growing season, of the maxima 
of temperature and of growth rates. These, maxima occurred about a month 
earlier at Oakland than at Easton— a fact that may be of significance in 
the comparative seasonal climatology of these stations, at least for the 
summer of 1914. 

6. The mean rate of leaf enlargement (as measured by the leaf-product) and 
also the mean rate of increase in dry weight, for the four-week periods of 
growth, followed seasonal marches that showed a secondary influence of the 
moisture conditions of the surroundings, as well as the primary one exerted 
by temperature. No apparent relation exists, however, in the data of the 
present study, betAveen the growth rates, on the one hand, and the data 
of either rainfall or evaporation, on the other; perhaps because the culture 
plants were protected from soil drought by auto-irrigation. The general 
moisture conditions of the surroundings were measured in terms of the ratio 
of rainfall to evaporation, however, and it is with reference to this ratio 
that the above-mentioned secondary influence of these conditions becomes 
apparent. This moisture influence appears to be most clearly shown by 
the growth rates for periods when the daily mean temperature was high 
(66° to 76° F.). Apparently it is the moisture conditions of the second half 
of the four-week period that are here influential. At the end of a month 
of growth from the seed the mature leaves are larger when the last two weeks 
of the period have been characterized by a high value of the rainfall-evapora- 
tion ratio than when these two weeks have been drier. This is related to 
the fact that the leaf development of the first month of growth mainly 
occurs in the latter half of the period. 

7. It appears that temperature was clearly the limiting condition (in the 
usual sense) for growth during the first two weeks, in practically all cases. 
During the second two weeks of growth, however, with exactly the same 
environmental conditions, the moisture relation (rainfall-evaporation ratio) 
appears in many cases to have been the limiting condition for growth, this 
being especially true, as has been remarked just above, when the tempera- 
ture was high. It thus appears that if two ])lants in different stages or phases 
of their development are exposed to the same fluctuations in environmental 
conditions, the hmiting condition for one plant during a succeeding period 
maA- be of an entirelv different nature from that for the other. This must 



Climatic Coxditioxs in Maryland 133 

be due to a difference between the internal conditions of the plants at 
different developmental stages. While this principle is so obvious as to 
appear not to require emphasis, it seems seldom to have been seriously 
considered in the literature of ecology and physiology. It must be consid- 
ered wherever standard plants are employed for the comparison of climates 

INTRODUCTION 

THE GENERAL PROBLEM 

The dependence of plants upon climatic conditions is almost self-evident, 
but the quantitative aspect of the relation between plant activities and cli- 
mate presents an exceedingly complex problem, the solution of which can 
not be expected for a very long time. Many investigators have attacked 
this problem, attempting to measure plant production or crop 3'ield in terms 
of the climatic conditions observed to be present during the growth period. 
This sort of research has usually resolved itself into attempts to correlate 
plant growth with one, or at most two, climatic factors — generally with 
temperature and rainfall, since these are both subject to marked geographical 
and temporal variations, and since both produce very evident effects upon 
the manner of growth of plants. The influence of temperature upon 
plant growth is marked and easily observed. Rainfall affects plant growth 
mainly in an indirect way, through its influence upon soil moisture. A num- 
ber of other factors, however, both climatic and non-climatic in character, 
are also continually exerting influences on plants, and plant growth is an 
expression of the effects of all these influences combined. Among these other, 
less frequently mentioned factors are: quality and intensity of sunlight, the 
evaporating power of the air, wind velocity, presence or absence of parasitic 
organisms, and many others. 

With so many variable factors entering into the equation that may be 
thought of as expressing the complex set of relations here suggested, no pre- 
cise correlation between plant growth and any single factor is to be expected. 
Nevertheless, temperature or rainfall does sometmies act as the most im- 
portant variable factor, producing the greatest variations in plant growth 
from year to year or from season to season in any given region or locality, 
as has been shown by several workers. Surprisingly close agreements were 
found Ijy Arctowski^ and by Smith,'* between crop yield and amount of rain- 
fall, and ]Merriam^ found a distinct general correlation between normal 
temperature conditions and the present distribution of plant and annual 



' Arctowski, Henryk, Studies on climate and crops: corn crops of the United States. Bull. Amer. Geog. 
Soc. 44: 745-760. 1912. 

■• Smith, J. Warren, The effect of weather upon the yield of corn. Monthly Weather Rev. 42: 78-92. 1914 
' .Merriam, C. Hart, Laws of temperature control of the geographic distribution of plants and animals. Na- 
tional Geog. Mag. 6: 229-238. 1894. 



134 FoRMAN T. McLean 

life on the Pacific coast of the United States. It does not appear, however, 
that such results are to be generally expected. A large number of factors 
are constantly varying in nature, and many of these are undoubtedly effec- 
tive to produce variations in the manner and rate of growth of plants. One 
single factor may be most influential, as rainfall in desert regions generally, 
but very many other conditions are also important for plant growth, and 
alterations in any of these effective conditions must surely exert some influ- 
ence on the rates. of the physiological processes in plants subjected to such 
alterations.^'' 

Not only do external conditions about the plants change, but the plants 
them.selves also change as time goes on ; they respond differently to the same 
external influences at different times in their life cycles or in different stages 
of their development. It follows that plant growth cannot be capable 
of expression in terms of climatic or other external factors, excepting by means 
of an exceedingly complex formula, which should involve all of the effective 
or controlling conditions. We are probably not yet even acquainted with 
all the factors that influence plants, nor do we know the action of those with 
which we are acquainted, so that attempts to establish what might be called 
a complete enviroimiental formula, — representing the total effectiveness 
of the surroundings to produce growth, maturation of seed, etc., for any 
given plant form, — must be postponed for a long time. 

The rate of growth of any given plant, however, is itself an expression 
of the sum total of all the effects of all the external conditions as these acted 
during the period of measurement.*^ Consequently, if it were possible to 
grow standard plants in different enviromiients, it should be feasible to 
measure and compare these environments in terms of their capacities to 
produce growth in the standard plants. 

Of course, such a procedure as that here suggested can be of but relatively 
little value in the interpretation of crop production, etc., unless the environ- 
mental conditions may be assorted into several groups which may be sepa- 
rately studied. To study them in this way involves the problem of main- 
taining certain groups of conditions sensibly alike for different standard 
plants, while other groups are varied. Thus the soil conditions, taken as 
a group, may be similar for a number of plant cultures, while the atmospheric 
conditions may be different. Differences in growth, etc., may then be con- 
sidered as due to the influence of the atmospheric complex, acting with the 
internal conditions that make up the nature of the plants used. 



*^ While the present paper was in press there appeared the following very important report of a physi- 
cal study of the relation of plant transpiration to certain environmental conditions: — 

Briggs, L. J., and H. L. Shantz, Daily transpiration during the normal growth period and its correla- 
tion with the weather. Jour. Agiic. Res. 7: 155-212. 1916. See also: Kiesselbach, T. A., Transpiration as a 
factor in crop production. Nebraska Agric. Exp. Sta. Research Bull. 6. 1916. 

* Livingston, B. E., Climatic areas of the United States as related to plant growth. Proc. Amer. Phil. Soc. 
52: 257-275. 1913. See especially page 258. 



■^ Climatic Conditions in Maryland 135 

This method of study should be valuable, of course, only when it may be 
assumed that the standard plants were alike at the beginning of the period 
of measurement. If they were not sensibly alike, interpretation of the ob- 
served differences in growth becomes practically impossible, for in such a 
case the argument is hopelessly complicated by the fact that the different 
internal- conditions initially effective in the various cultures enter into the 
logical analysis. To interpret the results obtained with plants that were 
unlike at the beginning of the experiment, an analysis of the internal con- 
ditions would first have to be made, and this presents far more difficulties 
than does the analysis of the relation between external conditions and the 
rate of growth. 

The mode of attack thus suggested was followed in planning the study 
here to be reported. '^ 

general plan of study 

The plants. To investigate the influence of chmatic conditions upon the 
growth of standard plants, it thus appears desirable to grow cultures of these 
plants under the different climatic complexes that are to be considered, and 
to treat all the cultures ahke in all other respects. Such like treatment of 
the cultures cannot be actually attained as yet, but an approach to this is 
possible. While it is to .be remembered that plants vary greatly among 
themselves, on account of various conditions as yet not well understood, and 
that they cannot at present be standardized in the same sense as thermom- 
eters and many other physical instruments, nevertheless this kind of study 
may be expected to elucidate some, at least, of the fundamental relations 
of plant growth to climatic conditions. 

The choice of standard plants for such investigations is rendered diffi- 
cult by several considerations. Since any given plant individual alters with 
the progress of time and according to its treatment, it is clear that plants 
that have been subjected to different treatments before the beginning of 
any comparative test are not to be employed. The standard plants must 
be considered as integrating instruments and, ideally, should be set at the 
"zero points" of their scales when the tests begin. This means that the plants 
employed must be in a stage of their development such that any differences 
that may have occurred in their past treatment have been registered in 
growth, internal change, etc., to as slight a degree as possible. Such a 
stage is presented in the seed; in this dormant phase the organism is but 
slightly affected by ordinary environmental variations. The seed was. 
therefore chosen as representing the zero point of growth in the study 
be here presented. 



' A short preliminary paper covering certain phases of this study has already appeared: McLean, Forman 
T., Relation of climate to plant growth in Maryland. Monthly Weather Rev. 43: 65-72. 1915. 



136 FoRMAN T. McLean » 

The species here employed were soy-bean (Glycine hispida Maximov.), 
Windsor bean (Vicia faba L.) maize (Zea mats L.) and wheat {Triticum 
sativum L). Only the results obtained with the first of these species, soy- 
bean, will be dealt with in the present paper. In order that the seeds ac- 
tually used for comparing the different sets of climatic curroundings might 
be as nearh^ alike as possible, they were all of the same strain and of the 
same crop, for each species employed. Furthermore, each lot was sorted, 
and all that appeared abnormal for any reason were discarded. 

The plants were grown from the seed in cultures in which all environmental 
conditions except those usually regarded as chmatic were controlled as fully 
as was practicable, and their rates of growth were used as a measure of the 
effectiveness of the whole group of surrounding conditions. The rate of 
growth is, of course, the amount of growth accomplished in a given unit of 
time, and it may be measured in terms of the amount of plant produced 
during the observation period. The quantities measured in this study were 
the size and weight of the plant produced, beginning with the seed, during 
periods of about two weeks and of about four weeks after planting. Divid- 
ing the result thus obtained by the number of days in the period gives the 
average daily rate of growth. The average of the rates of growth for several 
plants was taken as a measure of the comparative effectiveness of the climatic 
conditions as these tended to produce differences in plant activity at the 
several stations, during the growth period. These average growth rates 
were compared to values obtained by instrumental measurements of the 
climatic factors surrounding the plants during their period of growth. 

The cultures were started approximately every two weeks during the 
growing season, at each of the stations employed, and the growth obtained 
in each culture was determined after about two weeks and again after about 
a month. 

Measurement of the climatic conditions. No attempt was here made to 
secure a complete evaluation of the climatic conditions that affect plants. 
Indeed, such an attempt must be quite futile until much more is known 
about what climatic factors do effect plant growth, and how they act. The 
regular observations of the U. S. Weather Bureau, for temperature, rain- 
fall and duration of sunshine, with supplemental data bearing on soil mois- 
ture and evaporation, were brought together in various ways, for compari- 
son with the plant growth rates for corresponding periods. 

Nine cooperative stations of the U. S. Weather Bureau in Maryland 
were employed in this study. Their geographical locations are shown on 
the chart of figure 1. Of four stations on the Coastal plain, three (Easton, 
Princess Anne, and Coleman) are east of Chesapeake bay, and the remain- 
ing one (College Park) is much farther inland, near the borderline between 
the Coastal plain and the Piedmont plateau. Four stations are on the Pied- 
mont plateau, one (Baltimore) at its lower edge near Chesapeake bay, two 




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138 FoRMAN T. McLean 

(Darlington and Monrovia), in the hilly country north and west of Balti- 
more, and one (Chewsville) in the Hagerstown valley. The ninth station 
(Oakland) is on the tableland at the top of the Allegheny plateau. All of 
these stations excepting Oakland are at comparatively low elevations, less 
than 1000 feet above sea level. Oakland has an altitude of about 2500 feet. 

Without the most cordial support of the cooperative weather observers 
at the various stations this project could not have been successful, and the 
writer takes pleasure in here expressing his grateful appreciation of their 
very generous assistance. It is with much regret that mention must here 
be made of the death of Mr. J. S. Harris, the observer at Coleman. The 
other observers who cooperated in this work were: Mr. A. F. Galbreath, 
of Darlington; Mr. J. H. Lawson of Monrovia; Mr. D. P. Oswald, of Chews- 
ville; President H. J. Patterson, of College Park; Mr. H. Shreve, of Easton; 
Mr. J. R. Stewart, of Princess Anne; and Mr. R. E. Weber, of Oakland. 

Of the nine stations only two, Easton and Oakland, will be considered 
in the present paper, these being chosen to represent extremes of climatic 
conditions. Easton is on the flat coastal plain and has the characteristic 
humid, equable climate of the eastern shore of Chesapeake bay. It has an 
altitude of only 63 feet above sea level, The climate of Oakland, on the 
other hand, is typical of the moist but variable climate of the Allegheny 
plateau. The latter station lies on a gentle south slope, at an altitude of 
about 2500 feet. 

ACKNOWLEDGEMENTS 

The present studies were carried out under the auspices of the Maryland 
State Weather Service, and those of the Laboratory of Plant Physiology 
of the Johns Hopkins University, dui^ing the summer of 1914. The study 
was under the general direction of Prof. B. E. Livingston, to whom the author 
wishes here to express his great indebtedness for advice and aid, which alone 
made the work possible. The writer wishes also to acknowledge his in- 
debtedness to Prof. W. B. Clark and to the other members of the Board of 
Governors of the Maryland State Weather Serivce, for their esteemed 
support; to Dr. Oliver L. Fassig, Director of the Maryland Division of the 
U. S. Weather Bureau, for assistance in securing and compiling the weather 
data here used and in the selection of the stations at whicb the work was 
done; and to his associates, in the general project, Miss A. Hopping, Dr. J. W. 
Shive and Mr. E. S. Johnston, for cooperation and assistance. He also 
wishes to express his thanks to Mr. J. Calvert, who most kindly allowed 
the soil used in this study to be taken from his property, and to Prof . W. T. L. 
Taliaferro and to Mr, Grover Kinsey, who arranged for and supervised the 
shipment of the soil to the various stations. 



Climatic Conditions in Maryland 139 

METHODS AND EXPERIMENTATION. 

THE PLANTS 

The seeds and their treatment. The soy-bean used in this study was of 
the variety called '.'Peking/' with small black seeds. The plants are rather 
small but erect growers. The seed was of pure strain, obtained from the 
1913 crop of the Maryland Agricultural Experiment Station. Tests of it 
showed 98 per cent, of germination, when planted in moist quartz sand, 
in earthen pots in the greenhouse, at a temperature of about 58° to 71°F., 
during the early part of March, 1914. The seeds appeared very uniform. 
All small and all unusually large seeds, as well as any that appeared other- 
wise abnormal, were discarded. 

All the seeds were treated with carbon bisulphide vapor for one week, to 
destroy insects. Immediately after treatment, on March 27, 1914, the 
seeds were transferred to paraffined paper cylinders holding a quart, with 
tight-fitting covers. They were then stored in a laboratory locker, with a 
rather uniform temperature of about 72°F., until taken into the field for use. 

The seeds were planted 2.5 cm. (1 in.) deep, in moist soil in 15 cm. (6 in.) 
flower pots of the usual conical form, six seeds being planted in each pot. No 
records were kept of the dates of appearance of the seedlings above the soil 
at either of the two stations here considered, but such records are available 
for three of the other stations, Chewsville, Darlington and Monrovia. The 
average time required for the plants of soy-bean to appear above the ground 
at these three stations was 5.5 days from the date of planting. In the ma- 
jority of cases this time was from 4 to 6 days, the extremes recorded being 
3 and 11 daj^s. The slow rates of germination occurred during cold periods. 
The rapidity of germination will not be considered in the discussions that 
follow; the date of planting appears to furnish a more satisfactory initial 
point for calculating the growth rate than would that of the appearance of 
the seedlings above the soil. 

The growth measurements. Growth may be considered as essentially the 
process of development toward mature size, and it may be measured in 
several ways. In this investigation the growth rate of the soy-bean plants 
was considered from four points of view: (1) the rate of elongation of the 
plant shoot, (2) the rate of production of leaves, (3) the rate of development 
of leaves [in terms (a) of linear dimensions, and (b) of superficial area], 
and (4) the rate of increase in the dry weight of the plant. It is important 
that the measurements made in securing this sort of data be as rigidly uni- 
form as possible, to render the results comparable, and a regular method and 
order of measurement were therefore instituted and adhered to throughout 
the season. 

Each station was visited at intervals of approximately two weeks. At the 
time of the first visit after planting, the height of the stem, and the length 



140 FoRMAN T. McLean 

and greatest width of each leaf or leaflet were separately recorded for each 
plant. The stem height was measured from the soil surface to the base of 
the terminal bud; the base of the bud was here employed, rather than of 
the extreme tip, because the length of the bud is affected by the stage of 
development of the youngest visible leaf, and is therefore variable. The 
leaves were measured in regular order, from the base of the plant upward, 
each one being given a serial number in this order in the records. The leaf 
lengths were measured from the junction of blade with petiole to the tip 
of the blade. The width was measured at the widest part, and at right angles 
to the longitudinal axis of the blade. All linear measurements were made 
to the nearest millimeter. 

Approximately four weeks after planting all the measurements just men- 
tioned were repeated, after which the plants were cut off at the level of the 
soil. The leaves were immediately placed in a photographic printing frame 
and sun prints were made on photographic paper, from which the extent 
of the total leaf surface was later obtained, by means of a planimeter. Any 
plants that appeared to be unusual, because of accidental injury, and those 
that were markedly smaller or larger than the average were discarded. The 
plants of each culture were mailed to the laboratory at Baltimore, where 
they were dried for several days in the greenhouse, and then desiccated 
in an oven at 100°C., to constant weight. The dry weights were de- 
termined to 0.01 gram. These original dry weights included the cotyledons, 
when present, but these do not properly constitute a part of the growth of 
the plant after germination, and so the dry weights were afterwards corrected 
by determing the weight of the cotyledons in each case and subtract-ng this 
from the original amount recorded. This correction has been applied to 
all of the dry weight data employed in the present paper. 

As has been indicated, one culture of each of the four species used was 
started approximately every two weeks, at each station, and each culture 
was measured as described above. The results of these numerous measure- 
ments for soy-bean are suimnarized in tables I to IV, tables I and III refer- 
ring to the measurements of the plants when about two weeks old from seed, 
and tables II and IV referring to the second (and final) measurements of 
the plants, when they were about four weeks old from seed. These tables 
are similar in form, so that an explanation of tables I and II, for Oakland, 
will also serve to describe the manner of presentation of tables III and IV, 
for Easton. 

In table I the first four lines give general data. The different cultures 
are numbered in chronological order in the first line. The "date of planting" 
is given in line 2. "Number of plants" (line 3) refers to the number actually 
used in the measurements and thus excludes abnormal individuals which 
were discarded. 

The data of lines 5 to 13 present a summary of the plant measurements 



Climatic Conditions in Maryland 141 

obtained about two weeks after planting. "Age" (line 5) indicates the 
length of the period, in days, from the date of planting to the date of obser- 
vation, which was only approximate!}^ two weeks, the variations in this 
respect being due to the exigencies of the many trips necessary to each of 
the nine stations that were under observation. In reckoning these ages 
the date of planting was not included, the period beginning with the day 
after that date and ending with the date of observation. This age varies 
from 9 days in the case of culture 1 at Oakland to 17 days for culture 1 at 
Easton (table III). These differences necessitate that all of the data be 
ultimately expressed in the form of mean daily rates, or averages, in order 
that comparison of the different cultures may be possible. In all the plant 
measm'ements here given the data are averages per plant, the number of 
plants from which these averages are derived being given in the third line. 
Thus, the recorded stem height (Hne 6) for culture 1 (2.4 cm.) is the average 
height of the four plants of that culture, being expressed with the same de- 
gree of accuracy as in the case of the original measurements. The average 
daily increase in stem height (line 7) is obtained by dividing the average 
stem height (line 6) by the corresponding number of days (line 5), in each case. 
Line 8 shows the average number of full-grown leaves per plant, this num- 
ber being obtained by summing the number of leaves that had been developed, 
whether these were still present at the time of measurement or had previously 
died and fallen. Leaves that were approximately half -grown were considered 
as half leaves. The purpose of this enumeration is to get an expression of 
the stage of development of the plants; that is, to show how far the plants 
had progressed in their life cycle. Thus, a plant with three leaves mature 
and one half-grown is recorded as more advanced in growth than a plant 
with only three leaves, but appears as less advanced than a plant with the 
fourth leaf fully developed. Line 9 gives the average daily increment in 
the average number of leaves per plant, these data being obtained by di\'id- 
ing each number in line 8 b}'^ the corresponding number of days (line 5). 
The average leaf dimensions (l and w), given in lines 10 and 11, serve 
for comparison of the relative sizes of mature leaves in the different cultures. 
The unit of enumeration here employed was not the same as in the case of 
the number of leaves present (Hne 8). For the data of line 8 all leaves, 
whether mature or only partly grown, were considered, and the whole leaf, 
whether simple or compound, was taken as a unit; for the leaf dimensions 
I and w, on the other hand, only mature leaves were measured (or those 
very nearly mature, for small plants), and here the unit is a leaf or a leaflet, 
as the case may be. It is of no serious moment for the present purpose, 
whether the unit of surface is a leaflet of a. compound leaf (as the secondary, 
alternate leaves of soy-bean) or a simple leaf (as each of the initial pair of 
opposite leaves). The average "leaf-product," P (line 12), is the average 
per plant of the sum of the products of length nmltiplied by width for all 



142 FoRMAN T. McLean 

leaves, whether young or mature. These average leaf-products serve for 
general comparison of the relative leaf areas of the plants in the different 
cultures. The average daily increase in leaf-product (line 13) is obtained 
by dividing the average leaf-product (line 12) by the corresponding num- 
ber of days (line 5). The derivation of the soil moisture percentage (line 14) 
will be fully explained in connection with the discussion of soil environment, 
so that it will suffice to state here that each figure given is the average of the 
soil moisture data obtained for the culture in question during the period 
covered by the corresponding plant measm-ements. 

The data given in table II, also for Oakland, are the final measurements of the 
plants at the time of harvest, when they were approximately four weeks old. 
The data given in lines 1 to 13 and in line 19 correspond to the similar data 
of table I, fines 1 to 14, and need no further comment here. Lines 14 to 18, 
however, present data not considered in table I. "Leaf area" (line 14) 
is the average total area per plant of either leaf surface, the lower or the 
upper, but not of the total leaf surface, which would be doul)le the value 
given. This leaf area (A) was obtained by means of a planimeter, from 
photographic prints of the leaves, as has been mentioned. The average 
daily increment of leaf area (line 15) is obtained by dividing each number 
in line 14 b\^ the corresponding number of days (line 5). The average dry 
weight (fine 16) is the average weight per plant of only the top portion, 
excluding the parts beneath the soil surface, as has also been stated. The 
average daily increment in dry weight (line 17) is the quotient of the aver- 
age dry weight (line 16) divided by the corresponding age (line 5). The 

ratio of the average leaf-product to the average leaf area ( j ) is given in fine 

18. This will receive attention later. 

Appearance of plants. Tables I to IV give the measurements of the plants 
grown in the different cultures at the two stations, but they give no informa- 
tion about the manner of development of the plants or about the possible 
variable influences that may have affected them and that were neither 
measured nor controlled. It is desirable to consider here the general thrift 
and observed development of the plants, as well as some of the variable 
non-climatic factors that maj' have influenced the cultures. 

At the time of the first observations, about two weeks after planting, the 
cotyledons, which were still green and in healthy concfition, were always 
still attached to the young seedlings. The leaves were also bright green 
and appeared thrifty. During the first two weeks, then, the cotyledons 
being still attached, it may be supposed that the seedfings were subsisting 
upon the stored nutriment of the seeds, at least in part. 

At the time of the final observations, on the other hand, these conditions 
were different, but variable. At Oakland where the plants had grown 
rather slowly, the cotyledons were still green, still attached, and probably 



Climatic Conditions in Maryland 



143 





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PHYSIOLOGICAL RESE.VRCHES, VOL. 2, NO. 4, 
SERIAL NO. 14, FEBRUARY, 1917 



146 



FoRMAN T, McLean 



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Climatic Conditions in Maryland 147 

still furnishing material to the growing plants. At Easton, however, this 
state of affairs was the exception; it was encountered only toward the end 
of the season, when the plants had grown slowly. In most of the Easton 
cultures the majority of the plants had either lost their cotyledons, or the 
latter had turned yellow and were about to fall. Thus, these Easton plants 
were probably generally independent of the stored food supply of the seeds, 
at the time of the final observation. Furthermore, these plants were not 
as healthy in appearance as those in the corresponding stage at Oakland, 
their leaves being yellowish in color. 

It is possible that this unhealthy appearance of the largest and most 
mature plants at Easton may have been due to the absence of nutrifying 
bacteria on their roots. The soil used in these experiments had previously 
supported, among other plants, a wild vetch and is thus to be considered 
as provided with at least some nodule-forming bacteria, but these may not 
have been of the right kind for soy-bean. Root nodules appeared in the 
cultures of Windsor bean but not in those of soy-bean. The absence of 
root nodules in case of soy-bean, while it almost surely affected the growth 
rates of the plants, was not a variable factor, as this condition held 
throughout all of the cultures, and hence it should not interfere with the 
value of these cultm-es in comparing the effects of climatic conditions. No 
other apparently important non-cHmatic variable factors, which might seri- 
ously have affected the growth rates, were noted in the case of these soy- 
bean cultures. 

THE ENVIRONMENTAL CONDITIONS 

EXPOSURE OF instruments AND PLANTS 

Local and instrumental exposwe. The presence of trees or other objects 
in the near vicinity of the growing plants or of the cUmatological instru- 
ments may seriously influence such environmental conditions as the evaporat- 
ing power of the air, the intensity of sunshine, wind movement, etc., for the 
particular exposure in question. It is therefore desirable to have the plants 
and the instruments located near together, and exposed as similarly as pos- 
sible so that the growth of the plants and the readings of the instruments 
may refer to similar atmospheric conditions. It is also desirable that the 
culture and instrument locations be as much in the open as possible, in order 
that they may represent the general conditions of the I'egion as a whole 
and in order that the data obtained at the different stations may be studied 
in connection with the regional climatic conditions. 

The weather stations for both Oakland and Easton are situated in the 
open country. The Oakland station is on a south slope, about 1.6 km. 
(1 mi.) east of the town of Oakland, and near a public road, which is some- 



148 



FoRMAN T. McLean 



what dusty in dry weather. The general nature of the exposure of the 
plant cultures and instruments at Oakland is shown in figure 2. The cul- 
tures, the rain gage and the thermometer shelter were situated near together, 
about 12 m. (nearly 40 ft.) west of the greenhouses of H. Weber's Sons, and 
were fully exposed on all sides. The greenhouses, being only about 4.5 m. 
(15 ft.) high, did not produce serious wind obstruction, and they did not 
influence the light conditions appreciably. 

The Easton station was north of the residence of the observer, Mr. Henry 
Shreve, the general surroundings of the various instruments being shown 
in figure 3. The plant cultures were 1.2 m. (4 ft.) north of the rain gage 
and 18.3 m. (60 ft.) northwest of the dwelling. This position was protected 




Fig. 2. Culture enclosure and its general surroundings, at Oakland. 



on the south, southwest and west by large apple trees, about 15 m. (nearly 
50 ft.) distant, and by a low shed about 10 m. (33 ft.) distant on the north. 
The exposure is open toward the east. The crowns of the trees are too high 
to obstruct wind movement near the ground in a serious manner, but the 
plant cultures were shaded by the trees from about 3:30 until sunset, in 
June, and from 1 o'clock in September. The thermometer shelter stands 
under a tree west of the culture location and is about 6 m. (20 ft.) north of 
the house. Thus, all of the instruments at Easton as well as the plants, 
were somewhat protected, but not entirely screened, from strong winds 
from the north, west and south, and the plants were shaded in the late 
afternoon. 

The most pronounced difference between the exposure at Oakland and 



Climatic Conditions in Maryland 



149 



that at Easton lies in the amount of sunlight received. The Oakland plot 
was exposed to full sunHght throughout the day, while the Easton plot was 
shaded in the late afternoon. 

The climatological instruments (rain gage and thermometers) are of the 
standard pattern employed by the U. S. Weather Bureau, and have the 
standard exposure. The thermometers are in the usual shelter, 1.5 m. (5 ft.) 
above the ground, and thus do not experience all the temperature changes 
to which are subjected young growing plants near the soil surface. The air 
temperature around such plants is often greatly influenced by radiation 




Fig. 3. Culture enclosure (partly shown to the left of the rain gage) and its general 
surroundings, at Easton. 

from the soil, while thermometers with the standard exposure of the U. S. 
Weather Bureau are much less influenced in this way. The warming of 
the earth by isolation, and the rapid cooling on clear nights, by radiation 
into the atmosphere, subject low plants to extremes of temperature not usually 
recorded by the thermometers. It seems fair to suppose, however, that 
there is probably a fairly constant relation between the average daily marches 
of temperature for these two heights above the soil, in localities where the 
soils are similar in physical character, color and moisture-content, as is the 
case for the two stations here considered. The soils surrounding the plant 
cultures at both Oakland and Easton are rather heavy loams with brown 



150 



FoRMAN T. McLean 



top soil. They are somewhat strongly retentive of moisture, and are thus 
apt to be rather cold as compared to the air above them. It thus appears 
that the temperature conditions to which the culture plants were exposed 
at Easton and at Oakland may be safely compared, for the present purpose, 
by means of the thermometer readings obtained from the instruments in 
the elevated shelters. 

Plant exposure. The plant cultures were protected from accidental in- 
jury by enclosing the cultures at each station in a wire-covered frame. The 




Fig. 4. Interior view of culture enclosure, with top raised, showing irrigator pit, 
plunged pots and atmometer. (The spherical atmometers were being subjected to pre- 
liminary tests and their readings are not considered in the discussion.) 



form and arrangement of this is shown in figure 4. It was rectangular, 1.22 
m. (4 ft.) from north to south, 1. 83 m. (6 ft.) from east to west, 45 cm. (18 in.) 
high, and was surrounded with galvanized iron wire netting having meshes 
2.5 cm. (1 in.) in diameter. This cage was provided with a removable top 
consisting of a frame covered with wire netting, with meshes 5 cm. (2 in.) 
in diameter. A pit 61 cm. (2 ft.) deep, 1.22 m. (4 ft.) long from north to 
south and 41 cm. (16 in.) wide, was dug across the center of the enclosure, 
and was walled up with boards. The plant cultures, in ordinary flower 
pots, were arranged in two rows of six each just outside of the pit, one row 



Climatic Conditions in Maryland 151 

along each of its longer sides. The pots were plunged to such a depth that 
the soil surface of each pot was about level with that of the surrounding soil. 

As already stated, a new culture for each of the four plant species was 
started at each station approximately once every two weeks, and each cul- 
ture was continued for four weeks. Thus the culture periods overlapped, 
and there were regularly two sets, of four pots each, at each station. In 
addition to these, a third set of four pots, without plants, was constantly 
maintained at each station. The twelve pots, which were thus always pres- 
ent after the third visit to each station, were arranged in two rows running 
from north to south, a row of six on either side of the pit, and were so placed 
as to avoid as much as possible having the plants of the different cultures 
shade each other. The successive sets of four cultures each, were placed, 
two pots on each side of the pit, beginning with the north end, so that the 
younger plants were never directly north of the older ones. Also the cul- 
tures of the different species of the same age were arranged so that the more 
vigorous growers would not shade the slower-growing forms. Thus, Wind- 
sor bean was placed north of the corresponding soy-bean culture on the east 
side of the pit, and maize was placed north of the corresponding wheat cul- 
ture on the west side. 

The soil conditions. The same character of soil was used in all of the 
plant cultures. It was a rather light soil obtained from an untilled field 
near College Park, Md., and was of the soil type classified as Norfolk sand 
by Bonsteel.^ Its water-retaining power was found to be 43 per cent., 
on the basis of dry weight, by the Hilgard^ method, which employs a one- 
centimeter soil column. The top-soil was removed from a small area to a 
depth of 15 cm. (6 in.), and the soil thus obtained was thoroughly mixed 
and sifted. It was then placed in cloth sacks and shipped to the various 
stations, where it was stored in air-dry condition, in covered, water-tight, 
galvanized iron cylinders, until needed for use in the cultures. 

The soil containers for the cultures were ordinary porous clay flower pots, 
in form like the frustrum of a cone, being smaller at the bottom. Their 
inside dimensions were: top diameter, 15 cm.; bottom diameter, 9.5 cm.; 
height, 16 cm. The capacity of each pot was thus approximately 1980 
cc. of soil, when level full. However, in filling the pots, two auto-irrigator 
cups (to be considered below), each occupying about 72 cc, of volume, 
were also placed in each pot, and the soil was compacted so that it stood 
about 1 cm. below the top of the pot. The actual volume of soil in the pots 
when in use was approximately 1645 cc. 

In order to secure uniform soil conditions in the cultures, it was necessary 
not only to have soil of similar character for all cultures, but also to bring 



' Bonsteel, Jay A., The soils of Prince George's County. Publication of the Maryland Geological Survey. 
Baltimore, 1911. 

' Hilgard, E. W., Soils, their formation, properties and composition. New York, 1911. Page 209. 



152 FoRMAN T. McLean 

it into the same physical condition, so that it would retain its structure 
during the growth period of the plants and so that all cultures of the same 
age should have practically the same soil conditions. The very desirable 
condition of loose tilth could not be maintained in these cultures; since they 
were freely exposed to the weather and were visited only once in a fortnight, 
every rain must pack loose soil more or less, and heavy rains would saturate 
it completely. Therefore, to put the soil into a state of aggregation to be 
least altered by varying weather conditions, the soil was saturated 'with 
water immediately after it was put into the pots. This was accomphshed 
by plunging the pots of soil into a bucket of water, and allowing them to 
remain submerged until air bubbles ceased to rise. The pots were then 
set in position in the enclosures and allowed to drain. After the soil had 
settled, its surface was found to be one centimeter below the top of the pot, 
the soil mass having been compacted from 1736 to 1646 cc, or approximately 
5.5 per cent, of its loose volume. 

The soil moisture in the cultures was maintained by means of Livingston 
auto-irrigators.'" This device, as here used, consisted of two cylindrical 
porous clay cups (of the regular form supplied by the Plant World) 15 cm. 
long and 2.5 cm. in diameter. These were connected with each other and 
with the water reservoir by glass tubes in the form of an inverted J. The 
cups were placed vertically in the pot, their rubber-stoppered tops level 
with the soil surface, and were so arranged as to supply water to the soil 
against a pressure of 35 cm., or more, of water column. By this arrange- 
ment water is withdrawn from the porous cups by the capillary attraction of 
the water films in the soil about them. The difference between the pressure 
of the water in the cups and that of the soil films was adjusted in these 
experiments by placing the 1-gallon (nearly 4-liter) water reservoirs in the 
pits above described, 60 cm. below the surface of the soil, so that the 
water level in the reservoir, when the latter was full, was 35 cm. (14 in.) 
below the soil surface. The moisture content of the soil in the pots was 
thus maintained so that it was never less than about 10 to 13 per cent, (on 
the basis of dry weight), in which condition this particular soil was rather 
too wet than too dry for the best growth of the plants here studied. The 
soil was often moistened by rains, which sometimes increased the moisture 
content up to i+s maximum water-retaining power for the 15-cm. soil columns 
in the pots (approximately 23 per cent.). 

To prevent the movement of water and dissolved salts through the sides 
of the pot, between the culture soil and that surrounding the pot, the pots 
were painted on the outside. This coating proved inefficient, however, 
as the paint soon peeled off. The pots were later surrounded with strips 



"Livingston, B. E., A method for controlling plant moisture. Plant World 11: 39-40. 1908. 
Hawkins, Lon A., The porous clay cup for automatic watering of plants. Plant World 13: 220-227. 1910. 
Transeau, E. N., Apparatus for the study of comparative transpiration. Bot. Gaz. 52: 54-60. 1911. 



Climatic Conditions in Maryland 153 

of oilcloth, which appeared to be more satisfactory. Whatever water move- 
ment occurred through the pots, however,' was probably in the direction 
from the culture to the outside soil, as the soil in the pots was at all times 
more nearly saturated than was the soil around them. Thus there is little 
probability that the soil solution in the pots became greatly modified by 
the entrance of soil water from without. 

After preparing the pots and arranging the watering devices as described 
above, the pots were then allowed to remain fallow for about two weeks 
before planting. Thus the soil was fully drained out after the preliminary 
saturation, and had settled into a condition somewhat nearly approaching 
that of structure-equilibrium, before the seeds were planted. Care was 
taken to space the seeds uniformly, and to place them about equally distant 
from the auto-irrigator cups and from the sides of the pots, so that all should 
have, as nearly as possible under the conditions of the experiments, the 
same soil moisture conditions. 

At the time of planting, and at each fortnightly \'isit thereafter as long 
as each culture was continued, a soil sample was taken from each pot for 
the purpose of soil moisture determination. The method used was that 
described by Brown. ^^ A small cyhnder of soil containing about 22.5 cc. 
(about 30 grams, dry weight) was removed by means of a brass tube (cork 
borer), which was thrust into the soil vertically at a point midway between 
the two auto-irrigator cups, and about 3.75 cm. (1.5 in.) from each cup. 
Each soil sample thus taken represented a vertical section of the soil mass, 
of uniform diameter and extending from the upper surface to the bottom 
of the pot. The soil samples thus obtained were immediately transferred 
to heavily paraffined paper-pulp containers (which were serially numbered), 
and sealed in these with a paraffin seal. They were then sent, in paste- 
board maihng-tubes, to Baltimore, where the moisture content was deter- 
mined. Previous to the beginning of this experiment, the type of paraffined 
container here used (wliich is on the market for milk, etc.) was tested, as to 
its permeability to water vapor and it was found that the greatest loss from 
moist soil left sealed in such vessels for an entire week was not greater than 
about 0.1 per cent, of its dry weight. None of the field samples were ever 
left in the containers for a longer period than this, so that errors in the soil 
moisture determinations due to leakage may be regarded as neghgible. For 
hghtness in transportation and for general ease in handhng, these paper con- 
tainers were found very satisfactory. Larger ones of the same kind were 
employed for storing the stock of seeds. 

When the plants were removed from a pot (about six weeks after that pot 
was filled) the soil was discarded, and fresh soil from the stored supply- was 
always used in refilHng. 



u Brown, W. H., The relation of evaporation to the water content of the soil at the time of wilting. Plant 
World 15: 121-134. 1912. 



154 



FoRMAN T. McLean 





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156 FoRMAN T. McLean 

The results of the soil moisture determinations at Oakland and at Easton 
are shown in tables V and VI. These are given in detail, for all four species 
of each culture, to show the variations. The average of all determinations 
for each day of observation, together with the extreme deviations from the 
average, are given in the last column. In the cases where no data are given 
the auto-irrigating device was accidentally broken, and consequently the 
soil dried out. 

In these tables, each date given in the first column applies throughout 
the line in which it stands. Thus, on July 15, the second sample was ob- 
tained from culture 4, the first from culture 5, and the third from culture 3. 
The kind of plant grown, or to be grown, in the pots ig shown above, at the 
head of each column. 

The soil moisture contents of samples taken from different pots at the 
same station and on the same date are fairly consistent, as is shown by the 
column of extreme deviations, which generally show values between 1 and 
4, approximately, or from about 7 to 25 per cent, of the average soil moisture 
percentage. The values of the average soil moisture contents are quite 
variable, however; these averages range in value from 9.0 to 19.8 for Oakland 
(table V), and from 11.1 to 15.7 for Easton (table VI). These variations 
seem probably to have been due to differences in the climatic features that 
affect soil moistm'e, namely rainfall and evaporation. The irrigators simply 
kept the moisture content of the soil from becoming lower than about 10 
per cent. 

GENERAL MOISTURE CONDITIONS AND THEIR MEASUREMENT 

The water-supplying and water-withdrawing influences exerted by the 
atmospheric surroundings of ordinary terrestrial plants are to be grouped 
under two headings; precipitation and evaporation. The first of these 
(precipitation or rainfall) exerts its main influence upon ordinary plants 
indirectly, by increasing soil moisture and, consequently, the facility with 
which the soil may supply water to the plant roots. Precipitation was 
measured in the usual manner; the amount of moisture intercepted by the 
funnel of a standard U. S. Weather Bureau rain gage was measured each 
evening at sunset, and the result was computed as depth of rainfall in inches. 

In the present study interest in the environmental moisture conditions 
centers mainly about the influence exerted by the surroundings to alter the 
possible rate of water supply or the actual rate of water loss from plants. 
For ordinary terrestrial plants, which absorb practically all of their water from 
the soil in which they are rooted, the water supplying capacity of the surround- 
ings is mainly determined by the resistance offered by the soil to water-ab- 
sorption by plant roots.^^ This resistance varies with the character and 

12 For a more thorough discussion of this general topic, and for accounts of applications of this principle, 
see: Livingston, B. E., and Hawkins, Lon A., The water-relation between plant and soil. Carnegie Inst. Wash. 
Pub. 204: 3-48. 191.5. Pulling, H. E., and Livingston, B. K., The water-supplying power of the soil as indi- 
cated by osmometers. Ibid. 204: 49-84. 1915. 



Climatic Conditions in Maryland 157 

structure of the soil and with its moisture content. Since soil of the same 
character and - structure was employed in all of these experiments, the soil 
moisture content of these cultures may be considered as an approximate 
index of the power of the soil to supply water to plant roots. 

The environmental conditions most influential in determining the rate 
of water-loss from plants are the evaporating power of the air and the in- 
tensitj' or heat -equivalent of the absorbed radiant-energy, received from the 
sun.i^ The first of these was measured by atmometers, and the measure- 
ments will here be considered as indicating the degree of the tendency of 
the environment to promote evaporation from the plants. Radiant energy 
was not measured in this study, though a consideration of the available 
sunshine data will be presented farther on. 

The influence of the evaporating power of the air upon plants is twofold. 
It ma}' affect the water supply of the plants indirectly, by reducing the soil 
moisture, and it withdraws water from them directly, by evaporation (trans- 
piration). The evaporating power of the air was measured in these studies 
by means of cylindrical porous cup atmometers^^ located inside the culture 
enclosures, 45 cm. (18 in.) from the east end and 30 cm. (1 ft.) from the north 
side (fig. 4). The atmometer cups were provided with rain-correcting 
mercurj'- valves, and were mounted upon reservoir bottles of 500 cc. capacity. 
The reservoirs were placed upright in the soil, and at such a depth that 
the centers of the atmometer cups were from 28 to 32 cm. above the soil 
surface. These atmometers were refilled with distilled water and their 
water-loss was recorded at every fortnightly visit to each station. After 
every reading each atmometer was replaced b\' another that had just been 
standardized, and the used cup was returned to Baltimore, where it was re- 
standardized. All atmometer readings were reduced to terms of the Living- 
ston cylindrical standard, and they should thus be directly comparable 
with other measurements on the same basis. The spherical cups shown 
in figure 4 were employed only for preliminary tests of various makes of 
these instruments. They were not j-et perfected at the time this work was 
carried out. 

The effects of rainfall and evaporation upon soil moisture in the cultures 
here considered were much reduced by the use of the auto-irrigators, which 
gave, as has been stated, a minimum soil-moisture content of approximately 
10 to 13 per cent., on the basis of dry weight. The soil always appeared 
dark-colored and moist on the surface, and felt damp to the touch. Rain 



" Li\nngston, B. E., Light intensity and transpiration. Bot. Gaz. 52: 417-438. 1911. 

" Livingston, B. E., A rotating table for standardizing porous cup atmometers. Plant World 15: 157-162. 
1912. Other references are there given. 

, Atmometry and the porous cup atmometer. Plant World 18: 21-30, 51-74, 95-111, 143-149. 1915. 

Shive, J. W., An improved non-absorbing porous cup atmometer. Plant World 16: 7-10. 1915. 

Johnston, E. S., and B. E. Livingston, Measurement of evaporation rates for short time intervals. 
Plant World 19: 136-140. 1916. 



158 



FoRMAN T. McLean 



thus influenced the soil moisture content of the cultures only by increasing 
an already abundant supply of water. The general character of the effects 
of variations in precipitation and in the evaporating power of the air upon 
the soil moisture content in these partially controlled plant cultures may be 
seen from comparisons of the seasonal marches of these three factors, which 
are shown as graphs, for Oakland and for Easton, in figures 5 and 6, respec- 



5 H- 



M.Scc. 



-^toz 



hit 




MayZZ Junef IS JulyZ I's 3D %/3 26 SeH.II 

Rainfall, soil moisture and eva/^oratlon index, 
Oakland, /nd. 



2V 



nd.'i 



Fig. 



tively. The abscissas here represent time and season, and the ordinates 
(the values of the various data) are shown at the dates on which measure- 
ments were taken. The daily amounts of rainfall are represented at the 
bottom of each figure, as vertical lines, the relative lengths of which indicate 
the depths of rainfall. Soil moisture is given as percentage on the dry 
weight of the soil as basis. Each ordinate represents the average of all 



Climatic Conditions in Maryland 



159 



samples taken on the same day, these values being obtained from tables 
V and VI. Soil moisture data for June 4 at Oakland (fig. 5) are lacking, 
since the pots were saturated by rain at the time of observation, and the 
graph is drawn as a straight, broken line between the point for May 22 
and that for June 18. Evaporation is expressed as the average rate of loss 
from the standard cylindrical porous cup, in cubic centimeters per day, 
for each culture period of approximately two weeks. The duration of the 
period to which each rate applies is indicated by the length of the hor- 
izontal line drawn to the left of the corresponding point on the graph of 









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Rainfall, soil mo 


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isture and evajboralion index. 
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Fig. 6 



evaporation. Evaporation was not measured for the last two-week period 
at Oakland (Oct. 9, Fig. 5), since the atmometer cups had been broken 
by freezing. 

These graphs show an evident relation between rainfall and soil moisture, 
which is especially apparent in figure 5, for Oakland. Here the highest 
observed moisture content occurred on July 15, the day after a rain of 2.16 
cm. (0.85 inch), and the other high values, for August 26 and September 
11, were likewise due to rains, occurring in each case on the same day as 
that on which the soil sample was taken. These instances indicate that the 
effects of rain might persist in the culture pots for at least as long as two 
days. This is not alwaj^s necessarily the case, however, as is shown by the 



160 FoRMAN T. McLean 

observation for June 18. A rain of 1.27 cm. (0.5 in.) occurred on June 17, 
but the soil moisture content on the next day was somewhat below the 
average, being only 12.2 per cent. This rapid decrease in soil moisture 
was probably due to a high rate of loss by evaporation during the period 
between the cessation of the rain and the time of the observation. The 
general effect of high evaporation rates in reducing the soil moisture content 
of the culture pots is suggested in the seasonal marches shown by these 
graphs for Oakland. In every instance, excepting one (Aug. 26-Sept. 11), 
the graph for soil moisture slopes in the direction opposite to the direction 
of slope shown by the graph for evaporation for the corresponding time period. 
A similar relation is distinguishable in the graphs of figure 6, for Easton, 
though the relation between the two is here not so clear as in the other case. 
Soil moisture determinations were not made frequently enough, however, 
to show the detailed seasonal march of this condition in the cultures, and 
an exact study cannot be carried out in this connection. The evidence 
presented indicates clearly that the soil moisture content was greatly in- 
fluenced by both rainfall and evaporation, in spite of the employment of 
the irrigators. 

The probable effect of a given amount of rain upon the moisture con- 
tent of the soil in one of these culture pots may be approximately computed 
from the amount of water required to increase the moisture content from 
the value of the average maintained by the auto-irrigators to a value repre- 
senting the maximum water-retaining power of the soil-mass in the pots. 
The difference between these two latter values may he taken as the amount 
of water that, falling upon the exposed soil surface of the pot, would cause 
the maximum increase in soil moisture content. Run-off if not important 
in this calculation, as the side walls of the pots, extending a centimeter above 
the soil surface, effectively prevented its occurrence, excepting for the very 
heaviest rains, and drainage of water through the bottom of the pot should 
not be very pronounced until the soil mass had become moistened to above 
its maximum water-retaining power. The average dry weight per cubic 
centimeter of the soil, as calculated from fifty-two samples each containing 
about 22.4 cc, proved to be 1.4 g. The 1650 cc. of soil in each pot thus 
weighed about 2300 g., in the dry state. The maximum water-retaining power 
of the soil here used, as shown by the samples taken soon after rains, was 
nearly 23 per cent., on the basis of the dry weight of the soil, while the aver- 
age moisture content maintained in the pot between rains, by the irrigators, 
was about 14 per cent. Therefore, 207 cc. of water, added to the soil in 
its normal condition in the culture, would be all that could be retained 
against the downward attraction of gravity, for this amount corresponds 
to an increase in soil moisture content of 9 per cent., on the basis of dry 
weight. The area of the top of a pot of the type here used is approximately 



Climatic Conditions in Maryland 161 

200 sq. cm., so that a depth of rainfall of 1.03 cm.'^ (0.415 inches) received 
by a culture, after a period of drought, would be sufficient to bring the soil 
to its maximum water content, 23 per cent. On the basis of this calculation, 
the cultures at Oakland must have been saturated by at least ten storms 
during their combined growth period of twenty weeks, and those at Easton 
by seven or eight storms during their combined period of twenty-six weeks. 
Every light rain must also have exerted some influence upon the soil moisture. 
Under such varying conditions, the fortnightly soil moisture determinations 
become of little significance, except to show the general magnitude of the 
variations occurring during the season, and to indicate the relations holding 
between these and their external causes. 

As has been emphasized by Livingston and Hawkins, the environmental 
moisture conditions influencing plant growth, neglecting the influence of 
sunshine, may be 'considered as represented by the relation between the 
power of the soil to supply water to the plant roots and the power of the 
aerial environment to remove water by transpiration. Not being able as 
yet to measure the former of these two terms, but with due regard to the 
prime importance of the soil moisture content in determining the power of 
the soil to -deliver water to plant roots (especially in such cultures as those 
here dealt with, where the soil was all alike excepting for its moisture con- 
tent), an approximation of this value may be obtained by substituting the 
main component factor for the whole term of water-supplying power. This 
approximation may then be stated: the entire moisture relation of these 
plants is approximately represented by the relation between soil moisture 
content and the evaporating power of the air. This relation (expressed as 
a ratio) has been employed with considerable success by Shreve,!'' in studies 
of the relation between climatic and soil conditions, on the one hand, and 
plant distribution on the other. But such a ratio cannot be used in the 
present studies, since, as has been remarked, the soil moisture contents of 
the pots were not determined frequently enough to supply the needed data. 
From the information above set forth, and on general a priori grounds, a 
still less precise approximation may be attained by substituting in the place 
of the soil moisture content the main factor tending to increase this content, 
namely rainfall. Thus modified, the above statement becomes : the moisture 
relation of these plants may be approximately expressed as the ratio of rain- 
fall to the evaporating power of the air. This ratio, as will be seen at once, 
involves nothing but climatic conditions, and both of these two conditions 
were measured in the present study. Comparisons between such ratios 
for different time periods and for different localities should show, in a general 
way, the relative tendencies of the different cHmatic complexes to maintain 

15 This estimate is too high, since the retaining power of the soil was determined with a 1-cm. soil column 
and the column in the pot was much higher than this.— B. E. L., Ed. 

i« Shreve, F., Rainfall as a determinant of soil moisture. Plant World 17: 9-26. 1914. 

PHYSIOLOGICAL RESEARCHES, VOL. 2, NO. 4, 
SERIAL NO. 14, FEBRUARY, 1917 



162 FoRMAN T. McLean 

water in any water-absorbing substance (such as soil or a plant) . The ratio 
here proposed is the reciprocal of the ratio of evaporation to rainfall, as used 
by Transeau^^ which expresses the drying tendency of the climatic environ- 
ment. It is of course not important which form of ratio is employed, since 
one is the reciprocal of the other, but the writer finds it easier to think of 
an influence tending to moisten or to maintain moisture in an object than 
to think of an influence tending to withdraw water. 

In using rainfall as a measure of the general supply of moisture for plants, 
it is assumed that all of the water falling is effective to increase the moisture 
of the soil, which is the direct source from which plants absorb water. This 
assumption is not ordinarily strictly true, however, as has been clearly 
pointed out by Shreve, and as has been demonstrated also for the pot cul- 
tures here employed. As shown above, any rain in excess of 1.03 cm., 
occurring in a single shower, should have been without effect upon the soil 
moisture content in these cultures, this moisture content being already at 
its maximum. Thus, the rainfall-evaporation ratios as here derived are 
probably generally too high. 

TEMPERATURE CONDITIONS AND THEIR MEASUREMENT* 

Maximum and minimum temperature readings. The temperature data 
used in this study were all obtained from maximum and minimum ther- 
mometers read daily at sunset. The results of these readings are shown 
graphically in figure 7, in which the abscissas represent the successive dates 
of the daily observations, and the ordinates are the recorded maximum 
and minimum temperatures, in degrees Fahrenheit. The upper two graphs 
of figure 7 show the maximum and minimum temperatures recorded at Oak- 
land during the period of observation, and the lower two present the cor- 
responding data for Easton. 

The temperature conditions at the two stations, as shown in figure 7, 
exhibited large and irregular fluctuations, this being especially true of the 
western station. Both the daily range of temperature (within each 24 hour 
period) and the interdim-nal fluctuations (variations of the average conditions 
for different 24 hour periods) were greater at Oakland than at Easton. 

TEMPERATURE WEIGHTINGS AND INTEGRATIONS 

General discussion. It is obvious that this complex mass of temperature 
data must be greatly simplified before it may be compared to the plant 
growth measurements, which were obtained only at relatively long inter- 
vals. Two general lines of procedure have been proposed for simplifying 
such complex series of temperature observations, (1) the grouping of the 



" Transeau, E. N., Forest centers of eastern North America. Amer. Nat. 39: 875-889. 1905. 



Climatic Conditions in Maryland 



163 



temperature data into several classes and the computation of the lengths 
of the time periods during which each class obtains, and (2) the smiimation 
of the temperature data for certain seasonal periods. The method of tem- 
perature classes was developed by Koeppen^* and has been modified in 
various ways by later writers.^^ 

In making such temperatm-e summations as those just mentioned either 
the data for any given time period may be employed directly or they may be 
replaced, before the summation is performed, by measures of the tempera- 
ture efficiency, obtained by weighting each themperature magnitude in ac- 
cordance with its observed or probable effectiveness in promoting plant 




Fig. 7 

growth. Until more real information is at hand regarding the effects of 
different degrees of temperature upon growth processes, such methods are 
mainly of value in defining and comparing climates as such, for which pur- 
pose they have been employed by both of the writers mentioned above. 
Another somewhat similar method of simpljdng temperature data is 
that proposed by MacDougal.^'' The average hourly rates of growth are 



18 Koeppen, W., Die Warmezonen der Erde, nach der Dauei der heissen, gemiissigten und kalten Zeit und 
nach der Wirkung der Warme auf die organische Welt betrachtet. Meteorol. Zeitchr. 1: 215-226. 1884. 

* 1' See especially: Zon, R., Meteorological observations in connection with botanical geography, agriculture 
and forestry. Monthly Weather Rev. 42: 217-223. 1914. 

20 MacDougal, D. T., The auxo-thermal integration of climatic complexes. Amer. Jour. Bot. 1: 186-193. 
1914. 



164 FoRMAN T. McLean 

to be computed for some given plant form, for time periods during which 
the temperature ranges over specific intervals on the thermometer scale 
(intervals such as 40 to 45°, 45 to 50° F., etc.). Then the number of hours 
representing the duration of natural temperatures lying within each of the 
given ranges, for any given longer period (day, week, month, growing sea- 
son, etc.) is ascertained from thermograph records, and the product of the 
average hourly growth rate for each range by its number of hours is con- 
sidered as the total temperature efficiency for that range of temperature 
conditions. Such efficiencies are computed for all of the temperature ranges 
experienced during the time period considered, and the smn of all of these 
efficiencies represents the total temperature efficiency of the larger period, 
for promoting plant growth. The early literature referring to tempera- 
ture integrations is reviewed by Abbe,^^ and need not be considered further 
here. 

Laboratory studies carried out with plants have shown that plant growth 
rates are more or less directly related to temperature, the best study bear- 
ing on this matter being the recent one of Lehenbauer.^^ If plants are 
subjected to temperatures of different magnitudes maintained for definite 
time periods, all other conditions being kept approximately uniform, the 
growth rates are found to vary in a characteristic manner with the tem- 
perature conditions. Below a certain low temperature and above a cer- 
tain high temperature no growth takes place, these limits being termed the 
minimum and maximum temperatures for growth. If the growth rates 
for any plant form, when subjected to different temperatures, are expressed 
in the form of a graph, the temperature values being the abscissas and the 
growth rates of the plants being the ordinates, then the graph takes the form 
of a curve which shows a slow increase in rate of growth for temperatures 
just above the minimum, followed b}^ a rapid increase until the highest 
growth rate is approached. Then the increase in growth rate with rise 
in temperature becomes slow again until the highest point of the graph 
is reached (the latter point being termed the optimum), when the growth 
rate rapidly descends to zero with still further increase in temperature. 
Whether each of the temperatures experienced during the constantly vary- 
ing conditions ordinarily encountered out-of-doors will affect the growth of 
plants in tho same manner as does a similar maintained temperature, as 
usually employed in laboratory tests, may only be found out by actual trial. 
It may be tentatively supposed, however, that plants respond to the vary- 
ing temperatures of nature in a manner at least similar to that in which they 
react to maintained temperatures, and indices formulated on this basis 



21 Abbe, Cleveland, First report on the relation between climate and crops. U. S. Weather Bureau, BSII. 
36. 1905. Pages 169-343. 

22 Lehenbauer, P. A., Growth of maize seedlings in relation to temperature. Physiol. Res. 1: 247-288. 1914. 
Earlier references to the general subject are there given. 



Climatic Conditions in Maryland 165 

may be tested by comparing them to actual rates of plant growth, as ob- 
served out-of-doors. 

The use of direct summations of the daily, etc., temperature values is 
based on the supposition that, within the limits of the temperature range 
encountered in any given set of investigations, the rates of the plant process 
studied are approximately proportional to the indices of temperature ef- 
ficiency obtained by such summations. In other words, if a graph is con- 
structed to show the relation holding between temperature index values 
and growth rates, the indices of temperature being abscissas and the growth 
increments for corresponding periods being ordinates, then the graph should 
take the form of a straight line. Laboratory studies of the elongation of 
roots and shoots of seedlings indicate that the relation between growth rate 
and temperature is not really a linear one, but that the linear relation is 
approached excepting near the optimum and minimum. Thus, for what 
may be regarded as medium temperatures (intermediate between minimum 
and optimum) for a given plant form, temperature indices derived from sum- 
mations may be expected to approximate the actual efficiencies of the cor- 
responding temperatures. 

Many workers have sought a general law which would express the rela- 
tion of physiological processes in general (including plant growth) to tem- 
perature conditions. The chemical principle of van't Hoff and Arrhenius 
(which states that the velocity of many chemical reactions somewhat more 
than doubles for each rise of 18° F. in the temperature of the medium) has 
been found to apply quite well to several physiological processes, such as 
photosynthesis, respiration, germination of seeds, etc., but, of course, only 
within certain limits of temperature. The literature bearing upon the ap- 
plication of this principle has been reviewed by Livingston and Livingston, ^-^ 
who propose the use of temperature indices computed on the basis of the 
van't Hoff-Arrhenius law, employing a temperature coefficient of 2, and 
considering the rate of growth at 40° F. as the unit of temperature efficiency. 
As pointed out by these writers, such indices may not be expected to hold 
for all temperatures or for all plants. 

Temperature efficiencies based upon the application of the van't Hoff- 
Arrhenius principle take account of the fact that the relation thus supposed 
to hold between temperature and plant growth is not at all a linear one. 
Such efficiencies, expressed graphically, present a logarithmic curve. Accord- 
ing to the van't Hoff-Arrhenius principle an increase of one degree in temper- 
ature is more effective with relatively high temperatures, in accelerating 
chemical reactions, than is a similar increase with lower temperatures. As 
we have seen, laboratory studies of plant growth show that the increase in 
rate of growth is also accelerated with increase in temperature, up to a 



2' Livingston, B. E., and G. J. Livingston, Temperature coefficients in plant geography and climatology. 
Bot. Gaz. 56: 346-375. 1913. 



166 FoRMAN T. McLean 

certain limit. Out-door plants, subjected to varying conditions, may be 
expected to respond to temperature variations in a manner which accords 
more nearly with the van't Hoff-Arrhenius principle, than with the prin- 
ciple which supposes that the growth rates are proportional to the temper- 
atures themselves, above a certain point on the thermometer scale. 

Direct summations of daily means above Ifi° F. Direct summations of 
temperature readings appear to have considerable value, as a means for 
roughly comparing the temperature conditions affecting plants growing out- 
of-doors, and have therefore been computed for the various growth periods 
of the plant cultures used in the present investigation. To reduce the tem- 
perature readings on the Fahrenheit scale to effective temperatures for the 
growth of soy-bean, a daily mean temperature value of 40° F. was here taken 
as the minimum or zero for growth. No experimental data appear to be on 
record in the literature, upon the relation of the growth rate of soy-bean to 
temperature, but it has been found for many other plants that growth ceases, 
with falling temperature, at approximately 40° to 43° F., and therefore 40° 
may be taken as the minimum for the plants here used, without introducing 
the probability of great error. Having thus established a minimum, the 
effective temperature values were computed by substracting 40 from each 
successive daily mean. The remainders thus obtained for the successive 
days of each period were added, and the sum of the effective day-degrees 
thus obtained was treated as a measure of the temperature efficiency for 
the period. Such direct summations were made for all of the culture periods 
and for both stations here dealt with, and the results are given in tables IX- 
XII, which will be described below. 

Temperature summations obtained by use of the chemical coefficient. The 
chemical temperature efficiencies proposed by Livingston and Livingston 
may be computed from the data of ordinary fluctuating outdoor tempera- 
tures in at least three ways: (1) The efficiency index of the average tempera- 
ture for the whole period may be multiplied by the number of days in the 
period and the product thus obtained may be treated as the total efficiency. 
If the van't Hoff-Arrhenius law is applicable to the growth of plants sub- 
jected to the fluctuations of temperature encountered in the investigation 
here reported, then the temperature efficiency for each culture period, com- 
puted in the manner just described, should exhibit at least as close a correla- 
tion to the corresponding growth rate as does the corresponding direct sum- 
mation of eftective temperatures. (2) The indices for the successive daily 
mean temperatures may be summed for each period, thus giving due weight 
to the interdiurnal variations in temperature. (3) The indices for the 
maximum and minimum temperatures may be averaged for each day and 
the averages thus obtained may be summed throughout the period. In this 
way account is taken, not only of the interdiurnal variations but also of 
the daily range of temperature. Each of these three methods of computa- 



Climatic Conditions in Maryland 167 

tion has been employed for each of the culture periods and for each of the 
stations here considered, the results being summarized in tables IX-XII, 
which will be considered below. 

LIGHT CONDITIONS AND THEIR APPROXIMATION 

The present state of our knowledge regarding the relations to plant growth, 
of the different wave-lengths of radiant energ}^ that we term light, is still 
less satisfactory than is our knowledge regarding the temperature relation 
of plants. The portions of the solar spectrum that are most efiective in 
promoting certain plant processes are recognized in a general way, but the 
quantitative aspects of the problem here brought up have been but little 
investigated. Most of the studies that have been carried out in this con- 
nection have dealt with sunlight, and have treated the solar radiation as if 
it were constant in composition, if not in intensity, but of course it is not 
constant in either respect. Furthermore, while it is well known that only 
the light of certain portions of the solar spectrum is important for plant 
growth, the only data available for the evaluation of the sunlight for any 
locality for any period of time are rather rough, gfeneral estimates of the 
duration and of the heating. effect of the total radiation received. 

The duration of sunshine in hours per day (that is, the duration of light 
with a heating .effect above a certain very roughly defined minimum) is 
measured by the U. S. Weather Bureau only at its regular stations. The 
sunshine records for the cooperative stations, such as Oakland and Easton, 
ai'e only the local observer's ocular estimates of clear, partly cloudy and 
cloudy days. This latter form of record does not appear to be sufficiently 
precise, nor is it sufficiently detailed, to furnish data for a study of the rela- 
tions between plant growth and light conditions. Fortunately, however, 
complete instrumental records of sunshine duration are available for other 
stations in the vicinity of those here used. Thus the data for Elkins, West 
Virginia, have been here employed as if for Oakland, and the data for Wash- 
ington, D. C, and Baltimore, Maryland, have been averaged, the average 
being used as if for Easton. The general character of the weather at Oak- 
land is very similar to that at Elkins, the two stations being situated at simi- 
lar altitudes and about 48 km. (30 mi.) apart. Likewise, the weather con- 
ditions at Easton are generally not markedly unlike those at Baltimore and 
Washington, which are only about 50 and 75 km. (30 and 45 mi.) distant. 
The averages obtained by combining the records for Baltimore and for Wash- 
ington were compared to the local observer's records at Easton day by day, 
and the two sets of data showed very few disagreements as to the character 
of the days, whether clear or cloudy. 

All the sunshine data are presented in tables VII (Oakland) and VIII 
(Easton), to which reference will be made in the following paragraphs. 



168 P'ORMAN T. McLean 

For every culture period there are shown in these tables two approximations 
of the amount of sunshine available to the plants: (1) the average daily- 
duration of actual sunshine as derived from the average daily duration of 
possible sunshine for that period and latitude, and from the local observer's 
records, and (2) the average daily duration of actual sunshine recorded by 
the sunshine-recorder at Elkins (for Oakland), and the mean of the num- 
bers thus recorded at Baltimore and Washington (for Easton). 

In the case of the local records of the cooperative observers, days recorded 
as clear were treated as whole days of sunshine (column 4), partly cloudj^ 
days were treated as half-days of sunshine (column 5), and cloudy days 
were treated as without any sunshine (column 6). These days and half- 
days were summed for each culture period for each station, and the sum 
was expressed (column 8) as percentage of the total number of days in the 
3ulture period (column 3). Next, the average daily duration of possible 
sunshine for this region,^'' for each period (column 7), was multiplied by the 
corresponding observational percentage (column 8), to give an approxima- 
tion of the average daily duration of actual sunshine at the two stations con- 
sidered (table VII, column 10; table VIII, column 12). 

For each day of the Oakland season the percentage of possible sunshine 
duration was obtained from the records of the instrument at Elkins. These 
daily percentages were then averaged for each Oakland culture period, the 
resulting average daily percentages being given in table .VII, column 9. 
Each average daily possible duration of sunshine (table VII, column 7) 
was next multiplied by the corresponding average daily percentage (table 
\'II, column 9), to give an approximation of the average daily actual dura- 
tion of sunshine (table VII, column 11), for each culture period at Oakland. 

The percentages of possible sunshine duration for each Easton period 
were obtained from the instrumental records for Washington and for Balti- 
more (table VIII, columns 9 and 10), and these two data were then aver- 
aged for each period (table VIII, column 11). These composite percentages 
were each multiplied by the corresponding average daily possible number 
of hours of sunshine (table VIII, column 7), to give a number that is taken 
to represent the average daily actual duration of sunshine (table VIII, 
column 13) for each culture period at Easton. Since the cultures at this 
station were shaded from possible sunshine for approximately four hours 
each day, as has been noted, the data of column 13 require to be corrected 
so as to take this into account. This was done by subtracting from each 
number in column 13 four times the corresponding percentage in column 
11, since only this percentage of the four hours when the plants might 
have been in shade is to be considered as sunshine period. The numbers 
thus corrected are given in column 14. 



^* Derived from data in: Sunshine tables, U. S. Dept. Agric. Weather Bureau. 



Climatic Conditions in Maryland 169 

It appears that the numbers representing average daily actual sunshine 
as obtained by the first method (from local observers' records) are generally 
somewhat larger than those obtained by the second method (from instru- 
mental records for nearby stations). Which of these two series may be 
more nearly correct cannot be determined, however; both methods are 
very crude. The data derived by the second method (table Yll, column 
11; table VHI, column 14) will be employed where sunshine duration 
enters into the following discussions, but it is ' possible that the first 
method may prove of some value in climatic studies related to plant 
growth, in cases where ocular observations of cloudiness constitute the 
only available sunshine data. 

The sunshine records of the regular U. S. Weather Bureau stations, such 
as Elkins, Baltimore and Washington, are obtained by means of Marvin 
sunshine recorders, each of which consists of a pair of thermometers, so 
arranged as to record the lengths of the periods during which the heating 
power of the sunlight received is sufficiently great to maintain or surpass a 
certain difference in temperature between a blackened and a transparent 
glass bulb. The instrument thus records sunshine when the sun is shining 
brightly, but does not take account of differences in the absolute intensities 
of solar radiation received, except to show that the heating power is above 
the threshold value for the instrument. On account of the form of its in- 
clined cylindrical bulbs, this threshold value does not occur with the same 
hght intensity for different hours of the day nor for different seasons of tiie 
year. This kind of sunshine record is, however, the best that is obtain- 
able at the present time, and it may be supposed to have some possible 
value as indicator of favorable or unfavorable conditions for plant growth. 

The studies of Richter^^ indicate that photosynthesis in plants, and con- 
sequently the rate of formation of carbohydrates (which generally consti- 
tute the greater part of the dry substance of these organisms) is proportional 
to the amount of light energy actually absorbed by the leaves. From this 
it may be supposed that the sunshine data here considered may indicate, 
in a general way at least, the relative amounts of radiant energy available 
for plants during the different time periods, providing the quality of the 
sunlight does not vary greatly. 

Studies on the effect of shading upon plant growth, by Lubimenko-^ 
Combes," Rose,28 and others indicate that moderate variations in hght inten- 
sity may be accompanied by very great differences in plant growth. In most 



25 Richter, A., Etude sur la photosynthese et sur 1' absorption par la feuille verte. Rev. gen. Bot. 14: 1.51-169, 

211-218. 1902. 

26 Lubimenko, W., Production de la substance seche et de la chlorophyll chez les vegetaux superieures aux 
differents intensit^s lumineuses. Ann. Sci. Nat. Bot. IX, 7: 321^15. 1908. . , , 

=' Combes, R., La determination des intensitSs lumineuses optima par les vegetaux au divers stades de de- 
veloppement. Ann. Sci. Nat. Bot. IX, 11: 74-254. 1910. 

28 Rose, Edmond, L'energie assimilatrice chez les plantes. Ann. Sci. Nat. Bot. IX, 17: 1-110. 1913. 



170 



. FoRMAN T. McLean 





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171 



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172 FoRMAN T. McLean 

of these experiments, however, the conditions affecting the evaporating 
power of the air and the transpiration rates were not precisely enough 
measured or defined to make it certain that the differences in growth at- 
tributed to differences in hght were indeed entirely light affects. 

In addition to the sunshine records, the seasonal march of solar radiation 
in general is shown in the final columns of tables VII and VIII. These data 
are from Kimball's^^ graph of the maximum daily amount of radiation re- 
ceived upon a horizontal surface during each decade of the year, and is 
based on three years' record at Mt. Weather, Va. The latter station has 
an altitude of about 520 m. (1750 ft.) above sea-level, and is therefore fairly 
comparable to Oakland, Md. The main value of this Mt. Weather record 
for the present purpose, however, is to show the seasonal march of the aver- 
age maximum intensity per day, of solar radiation, for each culture period, 
for this general region. Thus, the average daily maximum radiation for 
culture period 3 for Easton appears to be more than twice as great as that 
for period 13 for the same station, and a similar relation holds, of course, 
for Oakland. These differences in the extreme values of total solar radia- 
tion per day are partly determined by the actual differences in radiation 
intensity at different seasons of the year, and partly by corresponding dif- 
ferences in the lengths of the daily periods of daylight. 

PRESENTATION OF WEATHER DATA 

All of the weather data here considered have been computed as summa- 
tions or averages for periods corresponding to the first two weeks (approxi- 
mately) of each full culture period, and also for each entire culture period 
of about four weeks, and the resulting values are given in tables IX-XII, 
which have already been mentioned. Tables IX and X refer to Oakland, 
tables XI and XII to Easton. Tables IX and XI show the data for the 
first two weeks (approximately) of each full culture period. The first line 
gives the serial numbers of the cultures, as heretofore used. Lines 2 and 
3 give the dates of the beginning and end of each two-week period. Line 
6 gives the direct summation of the daily mean temperature values in terms 
of degrees Fahrenheit above 40°F., for each period. The average daily 
mean effective temperature (line 7) is derived by dividing each number 
in line 6 by the number of days in the period (line 4). Line 8 shows the 
average daily range of temperature for each two-week period, this being 
the difference between the average daily minimum and the average daily 
maximum. 



29 Kimball, Herbert H., The total radiation received on a horizontal surface from the sun and sky at Mount 
Weather. Monthly Weather Rev. 42: 474-4S7. 1914. (See especially fig. 8, page 484.) 



Climatic Conditions in Maryland 173 

Lines 9 to 12 give temperature efficiency indices for each period, com- 
puted in three different ways, according to the general method proposed 
by Livingston and Livingston, employing the chemical temperature coeffi- 
cient. (1) To obtain the values given in line 9, the averge daily mean tem- 
perature for the two-week period in question was first found, and then the 
efficiency index corresponding to this value was obtained directly from the 
table given by Livingston and Livingston^" (page 366). To give the effi- 
ciency indices for the entire period (line 10), each number given in line 9 
was multiplied by the corresponding number of days (line 4). (2) To ob- 
tain the temperature efficiency values given in line 11, the mean tempera- 
ture for each day of the period in question was found by averaging its maxi- 
mum and minimum. The efficiency index corresponding to each of these 
daily means was then found from the Livingston and Livingston table, 
and these indices were finally summed. (3) The values given in line 12 
were derived in still another way. The efficiency index corresponding to 
ihe maximum and that corresponding to the minimum were found, from 
the table just mentioned, for each day of the period considered, and these 
two indices were averaged. The average daily efficiency indices thus 
obtained, were finalW summed. 

Lines 13 and 15 exhibit the total amount of rainfall for each two-week 
period, in centimeters and in inches. Line 14 gives the average daily rain- 
fall for the period, in centimeters. Line 16 shows the total evaporation 
for the period, in cubic centimeters, from the cylindrical porous cup atmome- 
ter, the readings having been first corrected to the Livingston cylindrical 
standard, while line 17 gives the average dail}^ evaporation for the period. 
Line 18 gives the value of the rainfall-evaporation ratio for each period, 
this being obtained by dividing each number in line 13 by the corresponding 
one- in line 16. Line 19 indicates the total number of hours of sunshine 
recorded by the Marvin sunshine recorder ai Elkins, W. Va. (representing 
Oakland) or the average of the similar records obtained at Baltimore and 
at Washington (representing Easton), for each period. Each value in line 
19 divided by the number of days in the period (line 4) gives the corre- 
sponding value in line 20, which is the average daily duration of sunshine 
for the period. 

Tables X and XII have the same form as tables IX and XI, and all of 
the description just given appHes also to these, excepting that the data of 
tables X and XII refer to the full culture periods of approximate!}' 4 weeks 
instead of to the first two-week portion of each full period. 



30 These efficiency indices were derived by the authors just mentioned, by considering the growth rate as 
unity at 40°F. and supposing that this rate doubles with each rise of 18° above 40°F. 



174 



FoRMAN T. McLean 







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177 



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PHTSIOLOGIC.VI/ RESEARCHES, VOL. 2, NO. 4, 
SERI.VL NO. 14, FEBRU.\.RY, 1917 



178 



FoRMAN T. McLean 



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182 FoRMAN T. McLean 

DISCUSSION 

The Oakland and Easton stations, as has been stated, were selected for 
comparative study in this paper because they may be taken to represent 
the extremes of the climatic conditions encountered in the Maryland area. 
In the discussion that follows, the various climatic features measured for 
these two stations during the summer of 1914, will be considered as environ- 
mental conditions presumably affecting the growth of the corresponding 
culture plants, and these paragraphs will present comparisons of plant fea- 
tures with climatic conditions for Oakland and for Easton, as well as com- 
parisons of the Oakland data with those of the other station. An attempt 
will be made to bring out some of the relations that obtained for the differ- 
ent observation periods at each station, between the soy bean plants and the 
concomitant and supposedly controlling conditions of their surroundings. 

THE FROSTLESS SEASON 

It has already been noted that the season of active plant growth proved 
to be much shorter at Oakland than at Easton. The frostless season at 
Oakland, for 1914, began after a severe frost on June 17, and ended with 
another severe frost on September 28. Thus the duration of this season 
was 103 days for Oakland. The experiments with which this paper deals 
were not begun at Easton until May 8, so that the last severe spring frost 
was not actually encountered at that station, but the records of the local 
observer show that this probably occurred April 11. The first killing frost 
in autumn occurred at Easton on November 7, so that the length of the frost- 
less season of 1914 was approximately 210 days for that station. At Oak- 
land the frosts mentioned were severe enough to kill so}^ bean seedlings in 
the general region, and the autumn frost actually did kill the plants in the 
experimental cultures. The same statement holds regarding the season 
at Easton, except that, since the last spring frost was not actually experi- 
enced by the cultures, its severity is merely inferred. The difference be- 
tween the lengths of the frostless seasons for Easton and Oakland, as here 
indicated (107 days), appears to be greater than is usual; as is- clear from 
Fassig's^^ data, the normal length of the frostless season for Easton is 201 
days and that for Oakland is 134 days, the normal difference being thus 
only 67 days. 



31 Fassig, Oliver L., The period of safe plant growth in Maryland and Delaware. Monthly Weather Rev 
42:152-158. 1914. 



Climatic Conditions in Maryland 183 

COMPARISONS between THE PLANT GROWTH VALUES AND CLIMATIC 

INDICES FOR OAKLAND WITH THOSE FOR EASTON, FOR THE 

ENTIRE PERIOD OF OBSERVATIONS 

Besides the length of the growing season, many other cHmatic characters 
are of course influential in determining plant growth, and some of these 
will now be considered, in relation to the corresponding growth characters 
exhibited by the culture plants here dealt with. 

The comparative efficiencies of the climatic conditions, as a whole, ex- 
pressed in terms of the growth of the soy-bean plants here employed, for the 
two stations and for the entire period of experimentation, are presented in 
the last column in tables I-IV. All of these are expressed both as average 
rates per day and as rates per period, excepting those for length and width of 
•mature leaves (l and w), which are given only as rates per period. These 
measurements of mature leaves were obtained in order to determine the in- 
fluence of the surroundings on the size attained by the leaves when mature, 
and these data were therefore not related to the rate of leaf expansion; they 
refer simply to the limit to this expansion set by the surroundings. There- 
fore average daily rates were not obtained in these two cases. On the 
other hand, those measurements of leaf length and width that were made 
for all leaves (whether mature or not) and that furnish the data for the 
leaf-products (P), require expression as m.ean daily rates. These leaf- 
products proved to be approximate measurements of total leaf surface 
per plant, developed during their respective periods. The data for two- 
week periods do not, of course, include actual measurements of leaf 
area, but in the case of the four-week periods the derived leaf-products 
and the actually measured leaf areas are both available for comparison 
(tables II and IV, lines 12 and 14). It is seen at once that these two 
series of values vary in the same direction across the tables. In order 
to investigate this parallelism each leaf-product in line 12 was divided by 

p 
the corresponding leaf area in line 14, thus giving the ratio- (line 18). 

Ji. 

If the leaf-products were always actually proportional to the corre- 
sponding leaf area, this ratio would have a constant value, and the act- 
ual values are seen, indeed, to be nearly constant. The average value 

P . 
of - in table II, for Oakland, is 1.34 and the greatest plus and minus varia- 

tions from this value are 7.4 and 5.2 per cent., respectively. Similarly this 
ratio average in table IV, for Easton, is 1.32, with maximum plus and minus 
variations of 7.5 and 6.1 per cent., respectively. It is therefore very clear 
that the leaf-product values (derived by measurements that do not involve 
any injury to the leaves) are to be considered as almost truly proportional 
to the leaf areas (which cannot be readily obtained without the removal of 



184 FoRMAN T. McLean 

the leaves from the plants). Since this conclusion holds so well in the cases 
where both leaf-products and leaf areas are available, it is probably safe to 
assume a similar relation in the other cases. Therefore, the leaf-products 
are here employed as approximate measures of leaf area both for the two- 
week and for the four-week periods. 

The average daily rates of plant growth for the whole season of these 
studies (measured by the various criteria) are given for both stations in 
columns 3 and 4 of table XIII, and these columns also include the data of 
length and width of mature leaves, all data being taken without change 
from the last column of tables I-IV. Column 5 of table XIII presents 
the ratios obtained by dividing each value for Oakland by the correspond- 
ing value for Easton. These ratios of the plant measurements thus express 
each average daily value for Oakland in terms of the corresponding average 
for Easton, and bring out the relations between the climatic conditions for 
the two stations, as indicated by the culture plants. 

The daily averages of the various weather data for the entire experimental 
season are also given, in columns 7 and 8 of table XIII, only one of the three 
temperature efficiency indices (the one corresponding to the daily mean, 
tables X and XII, lines 10) being given here. Column 9 gives the ratios 
obtained by dividing each climatic value for Oakland by the corresponding 
value for Easton. Thus, these climatic ratios express each one of the va- 
rious climatic values for Oakland in terms of the corresponding one for Eas- 
ton, as in the case of the growth values. 

The most evident feature brought out by the plant data given in table 
XIII is that the values for Easton are generally greater than t;-e correspond- 
ing ones for Oakland. This is strictly true of the values based on the shorter 
observation periods (plants about 2 weeks old from seed) and it is true of 
those based on the full culture periods (plants about 4 weeks old from seed) 
excepting in the case of the mean daily rate of increase in the total number of 
leaves per plant and in that of average length of mature leaves. The great- 
est difference between the average daily growth rates for the two stations 
occurs in the case of the rate of increase in the average of the products of 
leaf length by leaf width (leaf-product, P) for the shorter periods. For 
these younger plants the ratio of this rate for Oakland to that for Easton 
is 0.73. The greatest difference in these values based upon plants about 
4 weeks old, for the two stations, is shown by the rates of i;icrease in leaf 
area, dry weight of tops, and leaf-product, the ratios of these rates for Oak- 
land to those for Easton being 0.80, 0.86 and 0.89, respectively. The rate 
of increase in leaf area, measured either directly (by the planimeter) or 
approximately (by means of the leaf-product), and also the rate of increase 
in dry weight of tops, since these rates exhibit the greatest differences be- 
tween the two stations, may be considered as of probable value for com- 
paring the effectiveness of the climatic conditions for the growth of the cul- 
ture plants at one station with their effectiveness at the other. 



Climatic Conditions in Maryland 185 

The differences in the average daily values obtained for the climatic con- 
ditions, as here recorded for the two stations (table XIII, columns 6 to 9), 
are quite as great as or greater than the differences in the growth values. 
In comparing these it is to be remembered that the averages of the climatic 
data for the 2-week and 4-week periods are both computed from the same 
series of measurements, covering practically the same periods of time (these 
periods corresponding to the periods for which the plant data are computed), 
so that the average values derived from the two lengths of period are very 
nearly the same in this case, and they may be generally considered as prac- 
tically identical for the purpose of comparison with the daily averages of 
the plant data for the two stations. The daily averages of effective temper- 
ature (temperature in excess of 40° F.) and of the efficiencj^ index (derived 
from the chemical coefficient) are all about 84 per cent, as great for Oakland 
as they are for Easton, and a similar relation holds for the mean daily rate 
of evaporation, this rate being about 88 per cent, as great for Oakland as 
for Easton. The mean daily rainfall on the contrary is about 80 per cent. 
greater for Oakland than for Easton, and the rainfall-evaporation ratio for 
Oakland is more than double that for the other station. Similarly, the 
average daily duration of sunshine is about 43 per cent, greater for Oakland. 
Finally, the Easton station shows a higher average daily mean temperature, 
by about 5°F., but a smaller average daily range of temperature by about 
the same amount. 

It appears that the average of the climatic conditions experienced b}^ the 
successive cultures at Easton was more favorable for increase in leaf surface 
and in dry weight of tops (for both the 2-week and the 4-week periods) 
than was the corresponding average experienced by the cultures at Oakland. 
Excepting the average daily mean temperature, all of the major groups of 
environmental factors here considered were very different for the two sta- 
tions, and the daily averages given in table XIII do not indicate which factor, 
or group of factors, may have be-jn most influential in bringing about these 
differences in the growth rates. Of course it is possible that other factors 
than those considered in this study may also have been influential in pro- 
ducing the recorded differences in plant gro^vth here brought out. Some 
information as to the relative influences exerted by the different groups of 
climatic conditions that have been instrumentally measured will appear 
below. 



186 



FoRMAN T. McLean 



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188 FoRMAN T. McLean 

THE SEASONAL MARCH OF THE GROWTH RATES AND 

OF THE CLIMATIC CONDITIONS AT OAKLAND 

AND AT EASTON 

INTRODUCTION 

Any season in any locality may be considered as made up of a series of 
relatively favorable and relatively unfavorable periods, all of which con- 
tribute to the sum total of the season's influence upon the magnitude of 
the final yield of plants. The ultimate influence of any given portion or 
period of the growing season upon the final yield cannot generally be judged 
by the appearance of the plants at any time before the yield may actually 
be measured, for the thrift and rate of growth of a plant during any period 
of its development is a result of both present and past conditions. Thus, 
the rapid growth of a crop observed during a hot, dry week in midsummer, 
after a rain that terminated a drought period, is partly a result of the rain, 
which supplied water for the expansion of the tissues, but it is probably 
almost as much due to conditions effective within the drought period, dur- 
ing which the plants may have elaborated reserve materials, an abundance 
of which is quite as requisite for rapid growth as is an abundance of water. 

It is therefore necessary to study the seasonal march of each growth index 
and of each climatic index, as these vary throughout the season, for each 
station. These seasonal marches may be studied by employing the data 
recorded for either the two-week or the four- week periods; that is, by tak- 
ing the first two weeks or the first four weeks of growth from the seed as the 
short period of observation for the plants. Since these two methods do not 
bring out the same points, both will be employed below. The following 
sections are devoted to a study of the seasonal marches of the various plant 
and climatic indices, (1) based on the two- week periods, and (2) based on 
the four- week periods. 

SEASONAL MARCHES BASED ON PERIODS OF APPROXIMATELY TWO WEEKS 

Plant growth rates. Inspection of tables I and III brings out the fact that 
the three kinds of leaf measurements (length and width of mature leaves 
and leaf-product) nearly agree in the direction of their variation from period 
to period for both stations. On this account only one of these leaf meas- 
urements, the leaf-product, which shows the greatest variations, will be 
studied here. 

On the whole it appears that, of the five criteria of plant growth here 
employed, those of leaf-product and stem height show all of the essential 
points, and seem to be sufficient to exhibit the differences in growth rates 
from one period to the next. These alone will therefore receive attention. 

The variations in these two growth rates, from period to period, may be 



Climatic Conditions in Maryland 



189 



best represented by means of the graphs of figure 8 which give the average 
daily values of leaf product and stem height for Easton and for Oakland. 
The ordinates of the points on these graphs are the successive daily aver- 
ages per period of about two weeks, and the abscissas represent time and 
season, indicating the ends of the successive periods. Each graph thus 
shows the values of a single growth index as it varies through the season. 
The actual duration of the period for which any ordinate represents the 
average rate is indicated on the upper graph in each case (representing leaf 




Mayi 



25 Junes ZZ July(> 20 %.3 11 3/ Sej>t.N 28 Oct 1 1 2b 



Comparison of rates of grou/th of soi/ hean cultures, 2 weeks after Jstantin^, 
Easton and Oaldand. 



Fig. 8. P, leaf -product; E, stem height. The upper graph represents Easton, 

in each case. 

product for Easton and for Oakland, respectively) by the length of the hori- 
zontal line extending to the left of the given point. The points on the cor- 
responding graphs for stem height for Easton and for Oakland represent 
averages for the same time periods as are shown on the graphs of leaf-product. 
Each of these graphs exhibits an evident seasonal march, the daily aver- 
age per period increasing during a part of the season and then decreasing 
to low autumn values. This variation is more pronounced for Easton than 
for Oakland in both cases, which suggests that the seasonal changes in the 



190 FoRMAN T. McLean 

environmental conditions were more pronounced at the former station. 
There is no apparent similarity of detail, however, between the seasonal 
marches of the two graphs for either leaf-product or stem height. The maxi- 
mum in leaf-product for Easton occurs for the period from August 3 to 17, 
while that for Oakland occurs two weeks earlier. It must be noted, how- 
ever, that a secondary maximum for Oakland occurs for the period from June 
18 to July 2, and that this secondary maximum is almost as great as the 
other. It thus appears that the maximum rate of increase in leaf area may 
be expected to occur considerably earlier at Oakland than at Easton. In 
a similar manner the maximum rate of increase in height for Oakland is 
seen to occur about a month earlier than that for Easton. 

This apparent difference .in the seasonal position of the optimum period 
for the growth of these plants at the two stations, if it should prove to 
be a normal occurrence and if it be a characteristic difference betweeji 
lowland and mountain (or inland and coastal) localities for this region, 
might suggest interesting explanations of various agricultural facts; for 
example, the common observation that grains and other crops ripen 
earlier at high altitudes than at lower ones. If the progress of the life 
cycles of such crops should be associated with the seasonal march of 
what might be termed the plant-producing power of the climate, and if the 
decline that follows the attainment of the maximum in the environmental 
tendency to produce vegetative growth should prove to be important in 
accelerating flower and fruit production, (both of which suppositions are 
quite possible, as far as is known at present), then the normal date at which 
this optimum is reached in any locality might be of great significance in 
determining the tendenc}- of the cUmate of that region to produce early 
maturation of agricultural plants. Other more or less similar considera- 
tions might be mentioned, but no such questions as this can be answered 
without extensive investigation, continued through several years. 

Climatic conditions. The three sets of daily climatic data available for 
this study of the two-week periods (those of temperature conditions, those 
of moisture conditions, and those of light conditions), as given in tables IX 
and XI, are set forth graphically in figures 9 and 10, the former referring to 
Oakland and the latter to Easton. To faciHtate comparisons between the 
seasonal marches of these climatic averages and the corresponding marches 
of the two sets of values for average daily rates of plant growth for the fu'st 
two weeks from the seed, the gi'aphs of figure 8 are repeated in figures 9 
and 10. Thus each of the latter figures comprises five graphs, all referring 
to the same station and to the same series of two-week periods. The upper 
three refer to climatic conditions and the lower two to the corresponding 
rates of plant growth. These five graphs are comparable, in each case, as 
to direction of slope and as to the position of minima and maxima, but it 
should be remarked that the lengths of their ordinates are not directly com- 



Climatic Conditions in Maryland 191 

parable; the vertical scales employed are simply convenient ones and are 
quite arbitrary. 

The graphs of temperatm-e efficiency (T) exhibit comparatively regular sea- 
sonal marches in both figures. The ordinate value of this graph for Oakland 
(figure 9) increases from 24.3, for the second two-week period, to 27.6 for 
the third period, continues high during periods 4 and 5, and then decreases 
steadily to a final value of 20.7 for the ninth period. The ordinate value of 
the corresponding graph of effective temperature for Easton (figure 10) 
continues to increase for a longer time in the early part of the season. It 
attains a maximum value of 37.0 for the fifth period (which roughly corre- 
sponds to the fourth period at the other station), remains comparatively high 
(about 35.0) until the beginning of the ninth period (September 1), and 
then falls quite rapidly to a minimum of 12.8 for the thirteenth period, the 
last of the season. Thus the highest temperature efficiencies for Oakland 
occurred about two or three weeks earher than those for Easton, and the 
final dechne of this cHmatic condition began a month earlier at Oakland 
than at the other station. 

Another marked difference between the graphs of effective temperature 
for these two stations lies in the actual magnitudes of the various values. 
Not only are the values for corresponding time periods lower for Oakland 
than for Easton, but the differences Ijetween the highest and lowest value 
for Oakland is markedly smaller than the corresponding difference for Easton. 
The highest average effective temperature for Oakland is 27.7, while the 
highest for Easton is 37.0, and the minimum value for Oakland is 20.7, while 
the minimum for Easton is 12.8. The minimum value for Easton, however, 
occurred about a month after the Oakland season had been terminated by 
kilHng frost. From the above comparisons it appears that, in spite of the 
greater diurnal and interdiurnal variations in temperature recorded for 
Oakland, the seasonal march of the mean daily effective temperature here 
shows smaller differences between the extremes for the season than are ex- 
hibited for Easton. 

The graphs for sunshine (S) show a general downward slope throughout 
the season, for both stations, but both of the graphs are very irregular. 

The graphs for the rainfall-evaporation ratio f — j are also irregular for 

both stations. Both show low values for the beginning and end of the sea- 
son. That for Oakland shows a maximum for the two-week period ending 
July 30, and another maximum for the two periods ending August 26 and 
September 11. The maxima should indicate periods of most favorable 
moisture conditions and the minima should indicate those of relative drought 
The Easton graph for this ratio also shows two maxima (one for the period 
ending July 6 and the other for that ending August 31) Init these do not 
correspond, in the periods of their occurrence, with the maxima shown for 




^•i^^June^ IS July 2 15 30 fluq.lZ 26 Se^t.ll 2V 



Climatic conditions and ^roLuth of soy bean.,2. uieeh after jpUntCng, 

Oaklnnd. f^d. 



Fig. 9. T, temperature eflficiency; <S, sunshine duration; -, rainfall-evaporation 

ratio; P, leaf -product; K, stem height 

192 



Climatic Conditions in Maryland 



193 















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Climatic conditions and growt/j of soybean,! u/eeh after blanting. 


Easton. Md. 



Fig. 10. T, temperature efficiency; *S, sunshine duration; — , rainfall-evaporation 
ratio; P, leaf-product; H., stem height 



PHYSIOr.OGIC.VL RESE.^RCHES, VOL. 2, NO. 4, 
SERIAL NO. 14, FEBRU.VRY, 1917 



194 FoRMAN T. McLean 

Oakland, and the Easton graph is otherwise generally nearly horizontal 
It is noteworthy that the long-continued low values of tliis moisture ratio 
for Easton are markedly lower than the lowest values for the other station, 
also that the first maximum value for Easton is higher than either maximum 
for Oakland. It thus appears that the moisture conditions at Oakland were 
generally more favorable than those at Easton but that they were subject 
to greater extremes of fluctuation at the latter place. An interesting point 
is brought out when the graph for the moisture ratio is compared with that 
for sunshine duration, for the same station, in that a strong correspondence 
exists between these two graphs. Low values of the rainfall-evaporation 
ratio correspond generally to high magnitudes of the sunshine value, and 
conversely, for both stations. The moisture graph appears strikingly like 
what the sunshine graph would be if it were inverted. On days of prolonged 
sunshine the rainfall influence was low and the evaporation influence was 
high. 

Comparison of graphs of groivth 'rates with those of climatic conditions, for 
Oakland and for Easton. The graph of daily increase in leaf-product for 
Oakland (fig. 9) shows the same direction of slope as does the one for aver- 
age daily mean efl'ective temperature, from point to point throughout the 
season. To the extent to which the sunshine graph agrees in direction 
of slope, with that for temperature, it also shows the same directions of 
slope as do the corresponding sections of the graph for leaf-product. There 
appears to be no relation between the graph of leaf-product ' and that of 
the rainfall-evaporation ratio for Oakland. The graph of the average daily 
increase in height is similar to that of leaf-product for all periods except the 
sixth (ending Aug. 13). For this period the graph of leaf-product shows 
a marked decrease from the previous period, while that for stem height in- 
creases sHghtly. Here the graph for leaf-product follows the direction of 
the temperature efficiency graph, as in the case of the other periods. 

The graphs of effective temperature, of leaf-product and of stem height, 
for Easton (fig. 10), also show a general similarity in their seasonal marches. 
The sunshine and effective temperature graphs do not agree in trend for 
this station, as they do to a considerable extent for Oakland, and the graphs 
of growth rates show little tendency to conform to the variations in the 
direction of the sunshine graph. It may be noted here that the pronounced 
downward slope of the temperature graph does not begin until after the 
beginning of the corresponding decline in the graph for leaf-product. The 
temperature value continues high for about two weeks after the value of 
the leaf-product begins its downward trend. The decline in the graph of 
stem height begins even earlier than that in the graph of leaf-product. 

In conclusion, it may be said that both of the graphs of growth rate for 
soy-bean seedlings within the first two weeks after planting exhibit distinct 
seasonal marches, which verj^ closely resemble those of the corresponding 



Climatic Conditions in Maryland 195 

graphs for the temperature efficiency values. No apparent relation is 
noticeable between the two graphs of growth rates, on the one hand, and those 
representing the other two climatic factors, on the other. During the first 
two weeks after planting, the seedUngs germinated and their stems elon- 
gated rapidly. The first pair of leaves were formed, but usually were not 
fully developed, at the time of the first measurement (approximately two 
weeks after planting). Thus, growth during this early period consisted 
largely in stem elongation, and seems in these experiments to have been 
most influenced by temperature. Since most of this development must 
have been accompUshed at the expense of stored materials in the seed, and 
since an abundance of soil moisture was at all times supplied by the auto- 
irrigators, the principal factors affecting the rate of development of the 
plants were probably the rate of hydrolysis of stored materials and the rate 
of conduction of the latter from the cotyledons to the growing points. Since 
such processes, and also those of growth itself, are profoundly influenced 
by temperature, it is not surprising that correlations with temperature con- 
ditions are the only ones brought out by these graphs. It is not to be im- 
phed, however, that temperature was here the only influential factor, al- 
though it appears to be the most influential one, as far as these studies show. 

SEASONAL MARCHES BASED ON PERIODS OF APPROXIMATELY FOUR WEEKS 

Plant growth rates. As previously stated, each of the cultures was con- 
tinued for a second period of two weeks, at the end of which time the plants 
were again measured. The behavior of the plants during this latter period 
of two weeks was somewhat different from that during the first period. 
The leaves expanded much more rapidly during the second fortnight than 
during the first, so that a considerable leaf surface was finally developed. 
In most instances the cotyledons remained attached to the plants throughout 
the entire month of growth, but they became yellow in many cases and prob- 
ably usually became practically devoid of stored materials by the end of 
the month; in a few instances they fell from the plants before the time to 
harvest. Thus, with altered form and somewhat modified nutrition, the 
conditions influencing the rate of development of the plants might be ex- 
pected to produce different results in the second two-week period from those 
produced in the first. 

The most important daily increments obtained from these final measure- 
ments (tables 2 and 4) are shown graphically in figures 11 and 12, which are 
similar to figures 9 and 10. Two additional sets of measurements, not 
available for the two-week periods, were taken at this final observation, 
those of actual leaf area and those of dry weight. The data of leaf area, 
obtained from photographic prints, is to be regarded as a true measure of 
the extent of the leaf surface at the end of the four-week period. The leaf- 



196 FoRMAN T. McLean 

product was also obtained for the four-week periods, and, as has ah-eady 
been shown, these two measures of leaf area have a nearly constant ratio 
to each other, so that the relative rates of increase in leaf surface for the 
same plants during the two lengths of observation period may be directly 
compared by means of the leaf-product. Therefore the graphs showing 
rates of increase in leaf -product are; given in figures 11 and 12, and the 
graphs of actual areal increase are not presented. 

The data of dry weight of tops are especially important, since these are 
the only values obtained that furnish information on the approximate daily 
rates of accumulation of non-aqueous materials in the plants, and the 
graphs of average daily rate of increase in dry weight are included in figures 
11 and 12. 

In the case of Oakland the graphs of daily increase in leaf length, leaf 
width and total number of leaves, for these later measurements, were found 
to show a seasonal trend similar to that exhibited by the leaf-product, and 
they showed little variation in this respect for Easton. Hence these three 
criteria are not shown by the graphs of figures 11 and 12, and the graphs for 
daily increase in dry weight of tops, leaf-product and stem height are the 
only ones referring to plant growth given in these figures. 

Each of the graphs of average daily growth rates for the. four-week periods 
for Oakland (fig. 11), as well as each one for Easton (fig. 12), exhibits a 
seasonal march similar to that shown by the corresponding graph for the 
two-week periods. These three four-week graphs are similar in form for 
Oakland, being generally convex upward, but they all show concavity up- 
ward in the region of the fourth and fifth periods, this concavity being only 
slightly evident in the case of the graph for rate of increase in dry weight. 
The latter rises to its maximum for the third four-week period and then 
descends to the end of the season, with but a slight rise for the period 
ending August 13, while each of the other two shows two maxima. 

For Easton, the graphs of increase in dry weight and in leaf-product agree 
with the one of increase in leaf-product for Oakland in showing two maxima, 
with a concavity between them, and the graph of increase in dry weight 
for Oakland shows the first of these maxima, but the dates of these maxima 
are not the same for the two stations. While these two periods giving low 
values of the leaf-product for Oakland were those ending July 30 and 
August 13, the corresponding low values for Easton (of both weight and 
product) are for the periods ending July 20 and August 3. There thus 
appears to have been a difference of ten days in the dates corresponding 
to these low values of the graphs, the upward concavity occurring earlier 
at Easton. In this connection it must be remembered, however, that re- 
cords were obtained only at approximately two-week intervals, so that the 
exact dates corresponding to these two low values, or to the two maxima 
to which they are related, cannot be accurately fixed by the data at hand. 



Climatic Conditions in Maryland 197 

The graph of average daily increase in stem height for Easton shows a single 
maximum near the center of the season, for the sixth period. 

Climatic conditions. As has already been noted, the weather data for 
the four-week periods corresponding to the duration of the cultures repre- 
sented in the final measurements of the plants, are all derived from the data 
for the two week periods, to which they are naturally similar. The graphs 
of the three average daily values used for the fortnight periods (temperature, 
sunshine and rainfall-evaporation ratio) are shown for the longer periods 
in figures 11 and 12, these three graphs being again comparable as to direc- 
tion of slope and position of maxima and minima, but not as to the actual 
heights of their ordinates. The temperature graphs are of course smoother 
in this case than in that of the two-week data. The converse correspondence 
between the graph of the moisture ratio and that of sunshine duration is not 
as striking here as for the two-week periods, but is nevertheless apparent. 

Graphs of growth rates compared with those of climatic conditions, for Oak- 
land and for Easton. The graphs of plant growth (figs. 11 and 12) show no 
apparent relation to the marches of either rainfall or evaporation alone, but 
they do exhibit some interesting agreements with the march of the rain- 
fall-evaporation ratio, which is graphically shown in these figures. While 
several periods for Easton show low sunshine values as concomitant with 
high values of leaf-product and dry weight, and while one such correspond- 
ence is evident in the case of Oakland, yet the failure of this relation to be 
general and the somewhat unsatisfactory nature of the sunshine data ren- 
der the relation itself somewhat questionable. Of course it may sometimes 
occur that too strong sunshine may retard plant growth through the mois- 
ture relation, and thus bring about such a correspondence as that just men- 
tioned. The rainfall-evaporation ratio (representing the moisture relation) 
will receive attention below. 

For the data obtained two weeks after planting (figs. 9 and 10) there is 
no obvious agreement between the seasonal trend of the plant measurements 
and that of either sunshine duration or rainfall-evaporation ratio, but there 
is a general agreement between the seasonal march of effective tempera- 
ture (T) and that of stem height (H) for both stations, as has been pointed 
out. Similarly, the graph of leaf-product (P) for Easton shows a pronounced 
parallelism to that of effective temperature, but this agreement is not show n 
for Oakland. It thus appears that the temperature conditions were the 
main controlling factor in the first two weeks of stem elongation for both 
Oakland and Easton, but that this temperature control was not precise, 
some influence being exerted by other factors. It also appears that leaf 
expansion during the first 2 weeks of growth (measured by leaf -product) 
was rather definitely controlled by temperature conditions at the Easton 
station, but that other factors were not without influence upon this pro- 
cess at this station, these other factors constituting the main control at 
Oakland. 



198 



FoRMAN T. McLean 

















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CiimaiLC condittons ana growth of sou hean ^^ weeks after ^laniitig,^ 




Oakland, Md. 



Fig. 11. T, temperature efficiency; »S, sunshine duration; — , rainfall-evaporation 
ratio; P, leaf -product; W, dry weight; H, stem'^hcight 



Climatic Conditions in Maryland 



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Climatic conditions andqrowtti of soy bean ,tijueelis after f?lantinj. 
taiton,/^d. 







72 



Fio. 12. T, temperature efficiency; S, sunshine duration;-, rainfall-evaporation 
ratio; P, leaf-product; W, dry weight; H, stem height 



200 FoRMAN T. McLean 

In the case of the data obtained 4 weeks after planting (figs. 11 and 12), 
the graph of temperature conditions shows a general similarity to all three 
plant graphs, for both stations, but this agreement is only general. The 
plant graphs for leaf-product (P) and stem height (H) for Oakland agree 
with each other in showing two maxima (periods ending July 15 and Aug. 26), 
and the first of these maxima is also shown by the graph of dry weight (W). 
None of these maxima coincides in date with the single maximum of the 
graph of temperature efficiency. For Easton, the discrepancies of detail 
between the plant graphs and the the graph of temperature efficiency for 
the 4-week periods are fully as pronounced as those just mentioned. The 
graphs of leaf-product and weight both show two maxima (periods ending 
July 6 and Aug. 31), while this double maximum is barely indicated on the 
temperature graph. The graph of height fails to agree with the irregularities 
of either of the other two plant graphs and agrees only in its general seasonal 
march with the temperature graph. The four-week graph of sunshine shows 
no parallelism with any of the plant graphs in the case of either station. 

A study of the four- week graphs for Oakland (fig. 11) brings out the fact 
that the two maxima of each of the graphs of leaf-product and stem height 
coincide in time of occurrence with high points on the graph of rainfall- 
evaporation ratio and the depression between the two maxima on these 
growth graphs is also seen on the moisture graph. The second maximum of 
the moisture graph occurs a fortnight later than in the case of the graphs 
of leaf-product and stem height, but the upward slope of the moisture graph 
is slight for this fortnight. Also, the single maximum of the weight graph 
coincides in time with the first maximum of the graph of the moisture ratio. 
It thus appears that it is only for the beginning and end of the season that 
the direction of slope of the four-week plant graphs is generally the same 
as that of the corresponding temperature graph; during the middle portion 
of the season the plant graphs show a strong tendency to follow the direc- 
tion of the moisture graph, even where this graph differs radically from that 
of temperature. It may be said, for Oakland, that the four-week rates of 
increase in leaf-product and in stem height follow the four-week moisture 
ratio for that portion of the season when the four-week temperature efficiency 
value is above about 25. 

For Easton (fig. 12) the four-week data show a similar set of agreements 
and disagreements. In this case the height graph has nearly the same slope 
throughout as has the temperature graph, but the other two plant graphs 
show pronounced disagreements with the graph of temperature, excepting 
at the end of the season. It is suggested that these disagreements may be 
controlled by moisture conditions, but the two maxima of the four-week 
moisture graph do not synchronize with those of leaf-product and weight; 
the moisture ratio maxima occur a fortnight later. Here, again, it may be 
said that the plant graphs generally agree in direction of slope with the tem- 



Climatic Conditions in Maryland ' 201 

perature graph, from period to period, only when the temperature efficiency 
value is below about 25. 

From the two- week and four- week graphs of figures 9-12 it may be ten- 
tatively concluded (1) that the temperature relation is the main controlling 
factor for the growth rates based on the 2-week periods, no suggestion 
being apparent as to just what factors besides temperature may have been 
influential; (2) that temperature conditions are the controlling or limiting 
factor for growth rates based on the four-week periods, as long as the tem- 
perature efficiency values are not above about 25; and (3) that the mois- 
ture conditions (represented by the rainfall-evaporation ratio) seem to have 
exerted a marked influence upon the four-week growth rates during the time 
when the temperature efficiency values were above about 25. From the 
fact that the influence of the moisture conditions is not apparent for the 
two-week data and is apparent or strongly suggested for the four-week data, 
it appears probable that the moisture relation is relatively more important 
in the later stages of the development of the plants than in the earlier ones. 
If this supposition be true it may explain why the growth rates tend to fol- 
low the fluctuations of the moisture ratio as the plants become older. In 
such a case it should be the moisture conditions of the last 2 weeks of each 
of the four-week periods that are influential in determining the average 
growth rates for the four-week periods. To test this supposition, the four- 
week data of leaf-product and the rainfall-evaporation ratios corresponding 
to the last two weeks of each four-week period have been brought together 
for comparison in the graphs of figures 13 and 14, the former for Oakland 
and the latter for Easton. In these figures the four-week leaf-product 
graph (P) is the upper one, being reproduced from figure 11 or 12. The four- 
week temperature graph (T) lies next below, also derived from figure 11 or 
12. The third graph presents the two-week data of the rainfall-evapora- 
tion ratio (-) and is reproduced from figure 9 or 10. The length of period 

represented by each of the successive values of the leaf-product and of the 
moisture ratio is shown by a horizontal line extending to the left of each 
point on these graphs. 

Figures 13 and 14 indicate a pronounced parallelism between the four- 
week graph of leaf-product and the 2-week graph of the moisture ratio (for 
the last half of the four-week period), for both stations, thus furnishing evi- 
dence in favor of the supposition set forth above. It appears that the general 
rate of growth of these plants was influenced by temperature throughout 
the entire four-week period, but that the rate of leaf expansion was definitely 
influenced also by the moisture relations effective during the last 2 weeks 

of that period. . . 

It seems reasonable t.o suppose that as the plant grows larger its sensitive- 
ness to changes in its moisture surroundings increases; the greater expanse 



202 



FoRMAN T. McLean 



















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Comparison of growth in leaf l^rodud of soyhean uuiik 




temperature and rainfall- eva/yor ate on ratio. 
Oakland, All 



Climatic Conditions in Maryland 



203 




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25 JuneS 22 Juiyi 10 flug.li H 3/ Sept If 28 M// 26 Ntv.h 

Com fiariscn-ef groiuih in leaf firoduct of soy iean witA 

temferature and ralnfali- evaporafien ratio, 

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Fig. 14. P, leaf-product; T, temperature efficiency; — , rainfall-evaporation ratio 

hi 



204 FoRMAN T. McLean 

of surface should be accompanied by an increased requirement for water 
to supply that lost by transpiration. If this be granted it follows that the 
moisture conditions of the environment might be more than adequate for 
the early stages of development, while for later stages these same condi- 
tions might be limiting factors.^^ This condition of affairs is suggested by 
the relation just brought out, between the moisture ratio and the rate of in- 
crease in leaf expanse. These plants never experienced pronounced drought, 
for they were always abundantly supplied with soil moisture and the evapo- 
ration intensities of this region are never very high. There were slight 
fluctuations in the moisture conditions, however, and if these were to af- 
fect the plants at all this should occur when the requirement for water is 
greatest; namely, in the later stages of development, \\hen a relatively large 
surface is exposed to the air and sunshine. Furthermore, since rapid tran- 
spiration tends to deplete the water-content of leaves more than that of 
stems, it follows that leaf-growth should be retarded more than stem-growth, 
as the plant begins to experience a moisture deficit. The data of the pres- 
ent studies indicate that the culture plants were not markedly limited by 
the moisture conditions during the first two weeks of their growth from the 
seed, but moisture conditions did limit leaf expansion in the second two- 
week period from the seed. The moisture conditions here varied but little, 
but they seem to have varied enough to influence the plants when the lat- 
ter were most sensitive to them, and this influence became most markedly 
evident in that growth process (leaf expansion) which should be the most 
sensitive to these conditions. It may be added that this paramount in- 
fluence of the moisture conditions would be expected to become evident 
especially in periods of high temperature, when transpiration should be 
most accelerated. With lower temperatures (efficiency values below about 
25, according to these data) the moisture conditions should lose their power 
to influence the plant, and temperature should become the main control- 
ling condition. This is in accord with the facts above brought out. 

These various considerations may be summarized as follows. (1) Tem- 
perature exerts an influence at all times. (2) With low temperatures this 
influence is the controlling one, but with high temperatures the moisture 
relation becomes important. (3) When this occurs, it is the moisture con- 
ditions of the last 2 weeks that control the average rate of leaf expansion 
for a four-week period. 

A response in the growth rate to either one of two sets of climatic conditions 



'2 On the general theory of limiting conditions and its application, see the following: 

Blackman, F. F., Optima and limiting factors. Ann. Bot. 19: 283-295. 1905. 

Mitscherlich, E. A., Das Gesetz des Minimums und das Gesetz des abnehmenden Bodenartrags. Landw. 
Jahrb. 38: 537-552. 1909. Idem, Ueber das Gesetz des Minimums und die auf diesem ergebenden Schlussfol- 
gerungen. Landw. Versuchsstat. 75: 231-263. 1911. 

Smith, A. M., On the application of thetheoryof limiting factors to measurements and observations of growth 
in Ceylon. Ann. Roy. Bot. Gard.. Peradini ya 311: 303-375. 1906. 



Climatic Conditions in Maryland 205 

within the same extreme Hmits of environmental conditions, such as is in- 
dicated in this case for the four-week periods, was somewhat similarly de- 
monstrated by Smith (1906), for shorter observation periods, in a study of 
the relation of climatic conditions to the growth of the giant bamboo in Ceylon. 
As has already been mentioned, the two-week data of sunshine duration 
correspond in an inverse manner to the data of the moisture conditions, and 
it follows that wherever the latter conditions appear to have controlled 
growth it also must appear that sunshine duration was an influential factor^ 
The plants generally grew more with low sunshine values and less with 
high sunshine values. This somewhat unexpected observation seems to 
agree with the general physiological fact that plants actually grow most 
during the hours of darkness or of weak hght, and it points clearly to the 
conclusion that the controlling influences of sunshine in these studies was 
exerted through the water relation. It has been suggested^^ that physio- 
logical retardation of growth by light is really largely due to increased trans- 
piration during the daylight hours. If the sunlight influence noted above 
were exerted upon the photosynthetic process it would be expected to have 
the opposite direction to that actually indicated; the plants should grow more 
during periods having high sunshine values. But they are here found to 
grow less during such periods. 

In this general connection it is to be remembered, however, that the sun- 
shine data here employed are probably not as reliable as the other climatic 
data Of course other conditions than sunshine duration are influential 
in determining the value of the moisture ratio, but the relation here brought 
out emphasizes the apparent importance of sunshine as a factor m the water 
relation of plants. This matter will repay serious study whenever adequate 
methods for measuring sunshine may become available. 

CONCLUSIONS 

The study here reported was undertaken mainly to test, in a preliminary 
way certain newly devised methods for attacking the general problem of 
the relation of plant groxvth to climatic conditions. The results given in 
the preceding pages show that some of these new methods are of value and 
the data obtained by their means throw light upon the question of the in- 
fluence and relative importance of several different climatic features, as 
these affected the growth of the culture plants. 

The method here employed, of growing plants from seeds, as like as pos- 
sible, in pots of like soil, for approximately equal short time periods at dif- 
ferent stations, proved very satisfactory as a means of comparing climatic 
conditions for different localities and for different seasons of the year, as 

M Palladin, W., Pflanzenphysiologie. Berlin, 1911. P. 257. 



206 FoRMAN T. McLean 

these conditions influence plant growth. Such culture plants may be re- 
garded as integrating instruments for the measurement of climate. They 
are started from the resting condition, as seeds (in which state they may be 
considered as instruments set at the zero points of their scales), and the 
amount of growth accomplished after any given period of exposure may be 
taken as the summed result of all the environmental influences that have 
acted upon the plants during the period. Unlike many of the man-made 
contrivances employed as measuring instruments, the standard plants can- 
not be reset at the zero point after reading, but must needs be replaced by 
new and similar individuals in the resting stage. Errors due to individual 
variations in the plants, introduced by thus using a succession of standard 
plants, were not found to be excessive. 

The method here employed for soil moisture control, employing auto- 
irrigators, proved very satisfactory for the purpose, although the details 
of this technique are susceptible of considerable improvement. 

The methods by which the environmental conditions were measured in 
these studies were generally those in common use for similar purposes, and 
they require little comment here. The measurements of the evaporating 
power of the air, obtained with standardized cylindrical porous cup at 
mometers, taken with the ordinary precipitation records or measurements 
of rainfall gave a ratio of rainfall to evaporation that appears to be a 
very valuable measure of the environmental conditions as far as the water 
relation of the plants is concerned. Similarly, the records 'of daily maxi- 
mum and minimum temperature readings, as obtained from maximum and 
minimum thermometers of the type now in common use by the U. S. Weather 
Bureau, were found to be entirely adequate for all the needs of temperature 
measurement encountered in this work. Concerning the sunshine records 
as here employed, obtained with the Marvin sunshine recorder, it appears 
that such data promise to be of some value in indicating the relative influence 
of sunshine duration in determining the magnitude of the rainfall-evapor- 
ation ratio, but no other relation between this climatic feature and the 
growth of the plants was discoverable. 

Two methods of temperature summation were used in this study. One 
of these employs the direct summation of the daily mean temperatures 
above a certain assumed zero-point for growth, in this case above 40° F. 
The seconu is an indirect method, using temperature efficiencies derived 
from the application of the chemical principle of van't Hoff and Arrhenius, 
in place of the actual temperature readings above an assumed physiological 
zero. The two methods agree in showing a clear relation between tempera- 
ture and plant growth at both Oakland and Easton, but no evidence was 
brought out as distinctlj^ in favor of either of the two methods. 

The general conclusions of this study, regarding the relation of tempera- 
ture, moisture and light conditions to the growth of these soy bean seedlings. 



Climatic Coxditions in Marylaxd 207 

have been summarized in the Abstract, at the beginning of the paper. While 
these conclusions constitute only a beginning, it appears that further study 
along hnes similar to these may eventually develop a considerable knowledge 
of the relations here dealt with, and that this knowledge may furnish an 
important point of view for future climatological work. It promises, also, 
to be of practical value in connection with agriculture and forestry. In 
the planning of further studies, it must be borne in mind, however, that the 
problem is a very complex one, which cannot be expected to yield to any 
simple treatment. Nevertheless, it seems that climatology and plant 
physiology are now far enough advanced to warrant serious and sustamed 
attack upon this very important question of the relation of climatic condi- 
tions to plant growth and development. 

LITERATURE CITED 

\BBE C . First report on the relation between climate and crops. U. S. Weather Bur. BuU 36 1905. 
AHorowsKi. Henryk. Studies on climate and crops: corn crops of the United States. Bull. Amer. Geog. 

Soc. 44: 745-760. 1912. 
BLACKM4.N, F. F., Optima and limiting factors. Ann. Bot. 19: 283-295. 1905. 

BONSTEEL J. A., The soils of Prince George's County. Maryland Geological Survey. Baltimore. 1911 
Briggs, L. J., and H. L. Shantz. Daily transpiration during the normal growth period and its correlation 

with the weather. Jour. Agric. Res. 7: 15.5-212. 1916. 
Brown, W. H., The relation of evaporation to the water content of the soil at the time of wilting. Plant World, 

15: 121-134. 1912. , .. . _. j i i 

Combes, R., La determination des intensitfes lumineuses optima par les vegetans au divers stades de developpe- 

ment. Ann. Sci. Nat. Bot. IX, 11: 74-254. 1910. 
Fassig, O. L. The period of safe plant gro^vth in Maryland and Delaware. Monthly Weather Rev. 42. 152- 

158 1914 
Hawkins, Lon A., The porous clay cup for automatic watering of plants. Plant World 13: 220-227. 1910. 
HiLGARD, E. W., Soils, their formation, properties and composition. New York, 1911. , t,, . 

Johnston, E. S., and B. E. Livingston, Measurement of evapoiation rates for short time intervals. Plant 
' World 19: 136-140. 1916. . ^ o* t, v. 

KiESSELB.^CH, T. A., Transpiration as a factor in crop production. Nebraska Agnc. Exp. Sta. Research 

Bull. 6. 1916. ^ , ^ .. . „. „., , 

Kimball. H. H., The total radiation received on a horizontal surface from the sun and sky at Mount Weather. 

Monthly Weather Rev. 42: 474-487. 1914. 
KOEPPEN, W., Warmezonen der Erde, nach der Dauer der heissen, gemassigten und kalten Zeit und nach der 

Wirkung der Warme auf die organische Welt betrachtet. Meteorol. Zeitschr. 1: 215-226_ 1884^ 
Lehenbaxjer, p. a.. Growth of maize seedlings in relation to temperature. Physiol. Res^ 1: .47-.S8. 1J14. 
LmNGSTON, B. E., A method for controlling plant moisture. Plant World U: 39-40. 1908. 

, Light intensity and transpiration. Bot. Gaz. 52: 417-438. 1911. 

. A rotating table for standardizing porous cup atmometers. Plant World 15: 1^^ lb- ^^'- 

, Climatic areas of the United States as related to plant growth. Proc. Amer. Phil. Soc. 52: .o,-.7o. 

— -, itmometry and the porous cup atmometer. Plant World 18: 21-30, 51-74, 9.5-111. 143-149^ 1915^ 
Livingston. B. E.. and Lon A. Hawkins. The water-relation between plant and soil. Carnegie Inst. Wash. 
Pub. 204: 3-48. 1915. ^ j r ♦ i„„,. Rnt 

Livingston. B. E.. and G. J. Livingston. Temperature coefficients in plant geography and climatologj . liot. 

Gaz. 56: 346-375. 1913. . ,.„^^ 

LUBIMENKO. W . Production de la substance seche et de la chlorophyll chez les vegetaux superieure^s aux differ- 

ents intensit^s lumineuses. Ann. Sci. Nat. Bot. IX, 7: 321-41o. 1908. 
MACDOUG.U. D. T., The auxothermal integration of climatic complexes^ •^Tf'i^T; P MvfiS 1915 
McLe .n, Forman T., Relation of climate to plant growthin Maryland. Monthly Weather Re^ .43. 6a-.2. 191o. 
Merriam, C. Hart, Laws of temperature control of the geographic distribution of plants and animals. Na- 
tional Geog. Mag. 6: 229-238. 1894. „ j . t ^ 
MiTSCHERLicH.E.A.. Das Gesetz des Minimums und das Gesetz des abnehmenden Bodenertrags. Land^.. 

, Uebe^dls Gesetz des Min\mums und die auf diesem ergebenden Schlussfolgerungen. Landw. Versuchs- 

stat. 75: 231-263. 1911. 



208 FoRMAN T. McLean 

Palladin, W., Pflanzenphysiologie. Berlin, 1911. 

Pulling, H. E., and B. E. Livingston, The water-supplying power of tlie soil as indicated by osmometers. 

Carnegie Inst. Wash. Pub. 204: 49-84. 1916. 
RiCHTER, A., Etude sur la photosynthese et sur I'absorbtion par la feuille verte. Rev. gen. Bot. 14: 151-169, 211- 

218. 1902. 
Rose, E., L'enorgie assimilatrice chez les plantes. Ann. Sci. Nat. Bot. IX, 17: 1-110. 191.3. 
Shive, J. W., An improved non-absorbing porous cup atmometer. Plant World 16: 7-10. 1915. 
Shbbve, F., Rainfall as a determinat of soil moisture. Plant World 17: 9-26. 1914. 
Smith, A. M., On the application of the theory of limiting factors to measurements and observations of growth 

in Ceylon. Ann. Roy. Bot. Gard. Peradiniya 311: 303-375. 1906. 
Smith, J. Warben, The effect of weather upon the yield of corn. Monthly Weather Rev. 42: 78-93. 1914. 
Transeatj, E. N., Forest centers of eastern North America. Amer. Nat. 39: 875-889. 1905. 

■ — , Apparatus for the study of comparative transpiration. Bot. Gaz. 52: 54-60. 1911. 

ZoN, R., Meteorological observations in connection with botanical geography, agriculture and forestry. Monthly 

Weather Rev. 42: 217-223. 1914. 



VITA 

The writer was born June 27, 1885, at Colts Neck, New Jersey. He is 
the son of John Hull McLean, and Eliza (Taylor) McLean. He received 
his early school training in private schools, preparing for college at a public 
high school. He entered the Sheffield Scientific School of Yale University 
in 1904, graduating from that institution which honors in Forestry in 1907, 
and receiving the degree Ph.B. He attended the Yale Forest School the 
following year, and received the degree of Master of Forestry in 1908. In 
July, 1908, he entered the U. S. Forest Service as Forest Assistant, and was 
assigned to research work; first, !n the Bald Cypress swamps of the Gulf 
Coast (1908-1909), and later in the Rocky Mountains (1909-1913). In 
1912 he was placed in charge of the silvicultural investigations at the Utah 
Forest Experiment Station, as Forest Examiner. He was married on Octo- 
ber 27, 1913, to Mary Osborn Morford, of Red Bank, New Jersey. In the 
autumn of- 1913, the writer undertook the study reported in the preceding 
pages, under the auspices of the Maryland State Weather Service, and with 
the general direction of Prof. B. E. Livingston. 



LIBRARY OF CONGRESS 



002 595 278 9 



